Antenna array codebook with beamforming coefficients adapted to an arbitrary antenna response of the antenna array

ABSTRACT

Described herein are techniques where a transmitter for an entity of a wireless communication system, like a user equipment or a base station of a mobile communication network, is controlled by a codebook that has been established for a specific antenna array that is different from the transmitter&#39;s antenna array. The transmitter&#39;s antenna array may be an antenna array having an arbitrary configuration, e.g. the transmitter&#39;s antenna array may include a 1-, 2- or 3-dimensional antenna array of any array configuration. The specific codebook, e.g., a 2-dimensional discrete Fourier transform, 2D DFT, based codebook, is adapted to the actual or real antenna array of the transmitter by one or more characteristic matrices describing the configuration or nature of the transmitter&#39;s antenna array, thereby adapting the codebook, which has been established for the specific antenna array, for controlling the transmitter&#39;s antenna array so as to obtain a desired directional beam patterns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending InternationalApplication No. PCT/EP2018/052560, filed Feb. 1, 2018, which isincorporated herein by reference in its entirety, and additionallyclaims priority from European Application No. 17154487.7, filed Feb. 2,2017, which is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention concerns the field of wireless communicationsystems, such as a mobile communication network. Embodiments of thepresent invention relate to a multi-antenna technique using a precoderand a codebook with beamforming coefficients adapted to an arbitraryantenna response of a transmitter array. Further embodiments relate to adesign of a polarimetric codebook that may be adapted to a 3-dimensional(3D1 antenna response of the transmitter or receiver array, e.g., a2-dimensional discrete Fourier transform (2D DFT) codebook for apolarimetric antenna array response. At the receiver, the codebook maybe used for a direction estimation.

FIG. 1 is a schematic representation of an example of the wirelessnetwork 100 or wireless network infrastructure of the wirelesscommunication system of FIG. 1. The wireless network 100 may include aplurality of base stations eN131 to eNB₅ (eNB=evolved NodeB), eachserving a specific area surrounding the base station schematicallyrepresented by the respective cells 1021 to 102 ₅. The base stations areprovided to serve users within a cell. A user may be a stationary deviceor a mobile device. Further, the wireless communication system may beaccessed by Internet of Things (loT1 devices which connect to a basestation or to a user. IoT devices may include physical devices,vehicles, buildings and other items having embedded therein electronics,software, sensors, actuators, or the like as well as networkconnectivity that enable these devices to collect and exchange dataacross an existing network infrastructure. FIG. 2 shows an exemplaryview of only five cells, however, the wireless communication system mayinclude more such cells. FIG. 1 shows two users UE1 and UE2 (UE=userequipment), also referred to as user equipment (UE), that are in cell1022 and that are served by base station eNB₂. Another user UE₃ is shownin cell 102 ₄ which is served by base station eNB4. The arrows 1041,1042 and 104 ₃ schematically represent uplink/downlink connections fortransmitting data from a user UE1, UE2 and UE3 to the base stationseNB2, eNB4 or for transmitting data from the base stations eNB2, eNB4 tothe users UE1, UE2, UE₃.

Further, FIG. 1 shows two IoT devices 1061 and 1062 in cell 1024, whichmay be stationary or mobile devices. The IoT device 1061 accesses thewireless communication system via the base station eNB4 to receive andtransmit data as schematically represented by arrow 1081. The IoT device1062 accesses the wireless communication system via the user UE₃ as isschematically represented by arrow 1082.

The wireless communication system may be any single-tone or multicarriersystem based on frequency-division multiplexing, like the orthogonalfrequency-division multiplexing (OFDM) system, the orthogonalfrequency-division multiple access (OFDMA) system defined by the LongTerm Evolution (LTE) standard, or any other Inverse Fast FourierTransform (IFFT)-based signal with or without cyclic prefix (CP), e.g.Discrete Fourier Transform-spread-OFDM (DFT-s-OFDM). Other waveforms,like non-orthogonal waveforms for multiple access, e.g. filter-bankmulticarrier (FBMC), generalized frequency division multiplexing (GFDM)or universal filtered multi carrier (UFMC), may be used.

For data transmission, a physical resource grid may be used. Thephysical resource grid may comprise a set of resource elements to whichvarious physical channels and physical signals are mapped. For example,the physical channels may include the physical downlink and uplinkshared channels (PDSCH, PUSCH) carrying user specific data, alsoreferred to as downlink and uplink payload data, the physical broadcastchannel (PBCH) carrying for example a master information block (MIB) anda system information block (SIB), the physical downlink control channel(PDCCH) carrying for example the downlink control information (DCI),etc. For the uplink, the physical channels may further include thephysical random access channel (PRACH or RACH) used by UEs for accessingthe network once a UE synchronized and obtained the MIB and SIB. Thephysical signals may comprise reference signals (RS), synchronizationsignals and the like. The resource grid may comprise a frame having acertain duration, e.g. a frame length of 10 milliseconds, in the timedomain and having a given bandwidth in the frequency domain. The framemay have a certain number of subframes of predefined length, e.g., 2subframes with a length of 1 millisecond. Each subframe may include twoslots of 6 or 7 OFDM symbols depending on the cyclic prefix (CP) length.The PDCCH may be defined by a pre-defined number of OFDM symbols perslot. For example, the resource elements of the first three symbols maybe mapped to the PDCCH.

In a wireless communication system like the one depicted schematicallyin FIG. 1, multi-antenna techniques may be used, e.g., in accordancewith LTE, to improve user data rates, link reliability, cell coverageand network capacity. To support multi-stream or multi-layertransmissions, linear precoding is used in the physical layer of thecommunication system. Linear precoding is performed by a precoder matrixwhich maps layers of data to antenna ports. The precoding may be seen asa generalization of beamforming, which is a technique to spatiallydirect/focus data transmission towards an intended receiver.

In the following the downlink (DL) transmission in a mobile multipleinput multiple output communication system will be considered, i.e., thecommunication link carrying data traffic from a base station (eNodeB) toa mobile user equipment (UE). Considering a base station (eNodeB) withN_(Tx) antennas and a mobile user equipment (UE), with N_(RX) antennas,the symbols received at a particular instant of time in a DLtransmission at the UE yϵ

^(N) ^(Rx) ^(×1), may be written asy=HFs+n  (1)where Hϵ

^(N) ^(Rx) ^(×N) ^(Tx) denotes the channel matrix, Fϵ

^(N) ^(Tx) ^(×N) ^(s) represents the precoder matrix at the eNodeB, nϵ

^(N) ^(Rx) ^(×1) is the additive noise at the receiver, sϵ

^(N) ^(s) ^(×1) is the data vector transmitted by the eNodeB which hasto be decoded by the UE, and N_(s) denotes the number of data streamstransmitted.

The precoder matrix that has to be used at the eNodeB to map the data sϵ

^(N) ^(s) ^(×1) to the N_(Tx) antenna ports is decided by solving anoptimization problem that is based on the instantaneous channelinformation Hϵ

^(N) ^(Rx) ^(×N) ^(Tx) . In a closed-loop mode of communication, the UEestimates the state of the channel and transmits the reports, channelstate information (CSI), to the eNodeB via a feedback channel in theuplink (the communication link carrying traffic from the UE to theeNodeB) so that the eNodeB may determine the precoding matrix (seereference [1]). There are also occasions when multiple-layertransmissions are performed without feedback from the UE to determinethe precoding matrices. Such a mode of communication is called‘open-loop’ and the eNodeB makes use of signal diversity and spatialmultiplexing to transmit information (see reference [1]).

In the following, the closed-loop DL transmission mode will beconsidered. The CSI feedback sent to the eNodeB in the closed-loop modemay be of two different types: implicit and explicit. FIG. 2 shows ablock-based model of the Multiple Input Multiple Output (MIMO) DLtransmission using codebook-based-precoding in accordance with LTErelease 8. FIG. 2 shows schematically the base station 200, the userequipment 300 and the channel 400, like a radio channel for a wirelessdata communication between the base station 200 and the user equipment300. The base station includes an antenna array 202 having a pluralityof antennas or antenna elements, and a precoder 204 receiving a datavector 206 and a precoder matrix F from a codebook 208. The channel 400may be described by the channel matrix 402. The user equipment 300receives the data vector 302 via an antenna or an antenna array 304having a plurality of antennas or antenna elements. Further, a feedbackchannel 500 between the user equipment 300 and the base station 200 isshown for transmitting feedback information.

In the case of an implicit feedback, the CSI transmitted by the UE 300over the feedback channel 500 includes the rank index (RI), theprecoding matrix index (PMI) and the channel quality index (CQI)allowing, at the eNodeB 200, deciding the precoding matrix, and themodulation order and coding scheme (MCS) of the symbols transmitted. ThePMI and the RI are used to determine the precoding matrix from apredefined set of matrices Ω called ‘codebook’ 208. The codebook 208,e.g., in accordance with LTE, may be a look-up table with matrices ineach entry of the table, and the PMI and RI from the UE decide which rowand column of the table the optimal precoder matrix is obtained from.

The codebook designs in DL transmissions may be specific to the numberof antenna ports used for the transmission. For example, when two portsare used for the transmission, the codebook entries come from thecolumns of 2×2 unitary matrices with constant modulus entries (seereference [2]). For a 4-port transmission, the columns of householdermatrices B_(n)=I₄−2u_(n)u_(n) ^(H)/u_(n) ^(H)u_(n) may be used for theprecoder Fϵ

^(N) ^(Tx) ^(×N) ^(s) (N_(s)≤4 in this case), where u_(nϵ)

^(N) ^(Tx) ^(×1) is a vector with unit modulus entries, with n denotingthe codebook index (see reference [2]).

With explicit CSI feedback, there is no use of a codebook to determinethe precoder. The coefficients of the precoder matrix are transmittedexplicitly by the UE. Alternatively, the coefficients of theinstantaneous channel matrix may be transmitted, from which the precoderis determined by the eNodeB.

The design and optimization of the precoder 204 and the codebook 208 maybe performed for eNodeBs equipped with 1-dimensional Uniform LinearArrays (ULAs) or 2-dimensional Uniform Planar Arrays (UPAs) having afixed down-tilt. These antenna arrays 202 allow controlling the radiowave in the horizontal (azimuth) direction so that azimuth-onlybeamforming at the eNodeB 200 is possible. In accordance with otherexamples, the design of the codebook 208 is extended to support UPAs fortransmit beamforming on both vertical (elevation) and horizontal(azimuth) directions, which is also referred to as full-dimension (FD)MIMO (see reference [3]).

The codebook 208 in FD-MIMO is designed based on the array response ofan ideal UPA. The response of an antenna array, also referred to as‘array response vectors’, with N_(Tx) antenna ports is a complex-valuedvector of size N_(Tx)×1 which contains the amplitude gain and the(relative) phase shift induced or obtained at each antenna port of theantenna array 202 for a wavefront incident from a certain direction. Theresponse of an array is usually represented as a function of angle ofarrival or angle or departure. The codebook 208 used in the case ofmassive antenna arrays such as the ones FD-MIMO, is a set of beamformingweights that forms spatially separated electromagnetic transmit/receivebeams using the array response vectors of the array. The beamformingweights of the array are amplitude gains and phase adjustments that areapplied to the signal fed to the antennas (or the signal received fromthe antennas) to transmit (or obtain) a radiation towards (or from) aparticular direction. The components of the precoder matrix are obtainedfrom the codebook of the array, and the PMI and the RI are used to‘read’ the codebook and obtain the precoder.

The array steering vectors of an ideal UPA having identical antennaswith ideal antenna placement, e.g., antennas placed with infiniteprecision as dictated by the geometry, and omnidirectional radiationpatterns may be described by the columns of a 2-D Discrete FourierTransform (DFT) matrix (see reference [4]). Hence, for the codebook of2D UPAs 2D-DFT-based matrices may be used. 2D-DFT-based matrices aredefined for a scalable number of antenna ports, e.g., up to 32 antennaports per polarization/antenna orientation, or 64 antenna ports inco-polarized antenna arrays (see reference [3]).

The precoder matrices used in FD-MIMO may have a dual-stage structure:F=F₁F₂. Here, the matrix F₁ contains the beamforming vectors which aredefined by a 2D-DFT codebook, i.e., the matrix F₁ contains thebeamforming weights applied to each antenna port of the array to directthe radiation towards a particular direction. The matrix F₂ containscoefficients that select and/or linearly combine the 2D-DFT beams in thematrix F₁ to obtain a desired overall beam pattern. The matrix F₂ mayalso be used to perform co-phasing between different antennaorientations/polarization groups of the array (see reference [3]).

In massive antenna arrays, multiple antennas that are oriented indifferent directions may be placed at a particular position in thearray, i.e., there are P antenna ports at each position). Each of theantennas is sensitive to a particular polarization of the transmitted orreceived wavefront. As the orientation of the antenna defines thepolarization direction of the wavefront it is sensitive to, the terms‘antenna orientations’ and ‘antenna polarizations’ may be usedinterchangeably. However, in the following ‘antenna orientation(s)’ isused wherever applicable instead of ‘antenna polarization(s)’ so as toavoid confusion with wave polarizations that are also describedintroduced later. Considering a generic geometry of the array, thecomponents of the FD-MIMO-type two-stage precoder matrix for an arraywith N_(Tx) antenna ports per orientation, and P different antennaorientations among the antennas are, as shown, in the followingequation:

$\begin{matrix}{F = {{F_{1}F_{2}} = {{\begin{bmatrix}s_{1}^{1} & s_{2}^{1} & \; & \ldots & s_{D}^{1} & \ldots & \; & \; & 0 & \; & \; \\\; & \; & \vdots & \; & \; & \ddots & \; & \; & \vdots & \; & \; \\\; & \; & 0 & \; & \; & \ldots & s_{1}^{P} & s_{2}^{P} & \; & \ldots & s_{D}^{P}\end{bmatrix}\begin{bmatrix}c^{1} & \ldots & c^{D^{\prime}}\end{bmatrix}}.}}} & (2)\end{matrix}$

The matrix F₁ϵ

^(N) ^(Tx) ^(·P×D·P) has a block-diagonal structure with the arraysteering vectors for antennas with a specific orientation in each block.Each of the vectors s_(d) ^(p)ϵ

^(N) ^(Tx) ^(×1), d=1, 2, . . . , D and p=1, 2, . . . , P in F₁corresponds to a beamforming vector that steers the beam along certaind-th direction selected from D directions, using the antennas orientedin the p-th direction. The possible vectors for s_(d) ^(p)ϵ

^(N) ^(Tx) ^(×1) are the columns contained in the so-called ‘codebook’matrix of the array, which contains the steering vectors for variousangles of radiation.

The vectors c^(d), d=1, 2, . . . , D′ in F₂ϵ

^(D·P×D)′ are used to perform the beam selection or perform a linearcombination of beams. The combination/co-phasing of the beams may beperformed within and across different antenna polarizations in thismatrix. The variable D′ denotes the number of beams formed effectively.

To illustrate the use of the combining matrix F₂, the types of vectorsused in the matrix are provided along with the purpose they satisfy.

To select a specific beam out of the D steered beam directions in thematrix F₁ from both orientations/polarizations, the vector e^((d))ϵ

^(D×1), d=1, 2, . . . , D may be used which contains zeros at allpositions except the d-th position, which is one. For instance,

$\begin{matrix}{F_{2} = {\begin{bmatrix}e^{(3)} \\e^{(3)} \\\vdots \\e^{(3)}\end{bmatrix} \in {\mathbb{C}}^{{D \cdot P} \times 1}}} & (3)\end{matrix}$selects the beam steering direction corresponding to the third columnvector (in each of the block matrices along the diagonal) in the matrixF₁. Multiple beams may be selected using multiple columns, for e.g.,

$\begin{matrix}{F_{2} = {\begin{bmatrix}e^{(3)} & e^{(5)} \\e^{(3)} & e^{(5)} \\\vdots & \vdots \\e^{(3)} & e^{(5)}\end{bmatrix} \in {\mathbb{C}}^{{D \cdot P} \times 2}}} & (4)\end{matrix}$selects the beam directions corresponding to the third and fifth columnsin F₁. To perform beam selection while co-phasing between polarizations,a matrix of type

$\begin{matrix}{F_{2} = {\begin{bmatrix}{e^{j\;\delta_{1}}e^{(i)}} \\{e^{j\;\delta_{2}}e^{(i)}} \\\vdots \\{e^{j\;\delta_{P}}e^{(i)}}\end{bmatrix} \in {\mathbb{C}}^{{D \cdot P} \times 1}}} & (5)\end{matrix}$may be used, where the values δ_(p), p=1, 2, . . . , P are the phaseadjustments. Using vectors with more than one non-zero element, whileusing complex coefficients with varying amplitudes, means that multiplesteering vectors are combined while forming the beam.

The structure of the precoder in equation (2) considers that the numberof columns in each of the blocks, i.e., the number of beamformingvectors for each of the antenna orientations, is the same for eachblock. Such a structure is assumed for the sake of simplicity ofnotation and providing examples for F₂, and may be readily generalizedwith a different number of beamforming vectors for different antennaorientations.

The precoder structure in equation (2) and the structure of theindividual matrices F₁ and F₂ are generalizations of the precoderstructure in FD-MIMO for an arbitrary array geometry. For example,Release-13 FD-MIMO has been standardized for a uniform planar array asshown in FIG. 3. The configuration of a UPA may be represented as(N_(Tx) ^(H), N_(Tx) ^(V), P) where N_(Tx) ^(H) denotes the number ofantenna ports along a row of the UPA (in horizontal direction, hence thesuperscript ‘H’) per antenna orientation, N_(Tx) ^(V) denotes the numberof antenna ports across a column per antenna orientation, and Prepresents the number of antenna orientations in the array. Therefore, atotal of N_(Tx) ^(H)N_(Tx) ^(V)P antenna ports are present in the array.The values of P=1 and P=2 are used for co-polarized and dual-polarizedarrays, respectively. FIG. 3 shows a typical UPA used in FD-MIMO withP=2, along with the precoder structure.

The matrix F₁ϵ

^(N) ^(Tx) ^(H) ^(·N) ^(Tx) ^(V) ^(·2×D·P) has a block-diagonalstructure to separate the beams for the two polarization groups.Moreover, a Kronecker product model is applied to the steering orbeamforming vectors in F₁ to decouple them into separate horizontal andvertical steering vectors of the UPA. The steering vectors for eachdirection are taken from the codebooks for the respective directions.The codebook for the horizontal direction, in the UPA (the rows),denoted by Ω_(H), is given by a DFT matrix of size N_(Tx) ^(H)×M, whereM is the number of samples in the angular domain along the horizontaldirection. Similarly, Ω_(V) is the codebook for the vertical direction(columns of the UPA) and given by a DFT matrix of size N_(Tx) ^(V)×Nwith N being the number of samples in the angular domain in the verticaldirection. The matrices X_(H) ^(l), X_(V) ^(k), X_(H) ^(l)′ and X_(V)^(k)′ shown in FIG. 3 are formed by selecting a set of columns from theDFT matrices Ω_(H) or Ω_(V). Each of the matrices has the followingstructure:

$\begin{matrix}{{{X_{H}^{l} = {{\begin{bmatrix}x_{H}^{(1)} & x_{H}^{(2)} & \ldots & x_{H}^{(l)}\end{bmatrix}\mspace{14mu}{with}\mspace{14mu} x_{H}^{(i)}} \in \Omega_{H}}},{i = 1},2,\ldots\mspace{14mu},l}{{X_{H}^{l^{\prime}} = {{\begin{bmatrix}x_{H}^{(1)} & x_{H}^{(2)} & \ldots & x_{H}^{(l^{\prime})}\end{bmatrix}\mspace{14mu}{with}\mspace{14mu} x_{H}^{(i)}} \in \Omega_{H}}},{i = 1},2,\ldots\mspace{14mu},l^{\prime}}{{X_{V}^{k} = {{\begin{bmatrix}x_{V}^{(1)} & x_{V}^{(2)} & \ldots & x_{V}^{(k)}\end{bmatrix}\mspace{14mu}{with}\mspace{14mu} x_{V}^{(i)}} \in \Omega_{V}}},{i = 1},2,\ldots\mspace{14mu},k}{{X_{V}^{k^{\prime}} = {{\begin{bmatrix}x_{V}^{(1)} & x_{V}^{(2)} & \ldots & x_{V}^{(k^{\prime})}\end{bmatrix}\mspace{14mu}{with}\mspace{14mu} x_{V}^{(i)}} \in \Omega_{V}}},{i = 1},2,\ldots\mspace{14mu},k^{\prime}}} & (6)\end{matrix}$

The superscripts of the matrices differ depending upon the number ofcolumns in the matrix and the set of columns selected from the DFTmatrices; l=l′ and k=k′ means that the blocks for both polarizations inF₁ are identical (see reference [3]).

The second matrix F₂ contains coefficients that select and/or linearlycombine the array steering vectors in F₁ to obtain the desired beampattern. The choices for various functionalities such as single/multiplebeam selection and co-phasing between selected beams are as shown inequations (3) to (5).

The 2D-DFT-based codebook design used in FD-MIMO is advantageous as theoverall codebook may be divided into horizontal and vertical codebooksallowing for separate azimuth and elevation precoding, and as separatefeedback information is delivered to the eNodeB for the azimuth andelevation domains. Further, the 2D-DFT-based codebook for FD-MIMO allowsdescribing the array steering vectors of an ideal UPA by the columns ofthe 2D-DFT matrix (see reference [4]).

While a 2D DFT based codebook may be used as a codebook for uniformrectangular antenna arrays (URAs), since the antenna responses of theURA may be represented by an oversampled DFT basis vectors, a 2D-DFTbased codebook may not be used for an arbitrary shaped antenna array asthis may result in undesired directional beam patterns.

In other words, while a specific codebook for one antenna array may beused for controlling a specific antenna array (different from the oneantenna array), this specific codebook may not be suited for controllinganother or arbitrary antenna array. Thus, to achieve an accuratebeamforming and/or null steering for a realistic arbitrary antenna arrayat the transmitter, the codebook has to be adapted to the real ormeasured antenna array response of the transmitter. This means thatdifferent codebooks need to be designed for the different antenna arraysused. However, such antenna array-dependent codebook designs are notdesired as the codebooks are also dependent on the antennaconfiguration, such as the number of antenna ports, or an orientation ofthe antennas in the antenna array.

US 2016/173180 A1 describes systems and methods for a two-dimensionaldiscrete Fourier transform based codebook for elevation beamforming. Thecodebook supports single stream codewords and multistream codewords. Thetwo-dimensional discrete Fourier transform based codebook is generatedby stacking the columns of the matrix product of two discrete Fouriertransform codebook matrices. The codebook size may be flexibly designedbased on needed beam resolution in azimuth and elevation. A bestcodebook index is selected from the generated two-dimensional discreteFourier transform based codebook. The selected codebook index isprovided in a channel state information report. The channel stateinformation report is transmitted to a base station.

It is an object of the present invention to provide an approach whichavoids the need to design different codebooks for different antennaarrays used in respective transmitters and which achieves an accuratebeamforming and/or null steering for an actual antenna array used at atransmitter.

SUMMARY

According to an embodiment, a transmitter may have: an antenna arrayhaving a plurality of antennas for a wireless communication with one ormore receivers; and a precoder connected to the antenna array, theprecoder to apply a set of beamforming weights to the antenna array, theset of beamforming weights selected from a codebook to form by theantenna array one or more transmit/receive beams or nulls pointing inselected directions; wherein the codebook includes a plurality of setsof beamforming weights for a plurality of directions, wherein thebeamforming weights in the codebook are based on a first antenna arrayresponse matrix, the first antenna array response matrix defined by asecond antenna array response matrix and one or more characteristicmatrices, wherein the first antenna array response matrix contains, fora plurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix contains, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from theantenna array, and the one or more characteristic matrices describingone or more characteristics of the antenna array, and wherein the firstantenna array response matrix is modeled using the second antenna arrayresponse matrix and the one or more characteristic matrices, each of thefirst array response vectors being a product of a characteristic matrixand a corresponding second array response vectors.

According to another embodiment, a receiver may have: an antenna for awireless communication with a transmitter; and a signal processor toreceive and process a radio signal received at the antenna via a radiochannel; wherein the receiver is configured to construct a codebookincluding a plurality of sets of beamforming weights for a plurality ofdirections, the beamforming weights in the codebook being based on afirst antenna array response matrix of an antenna array of thetransmitter, and the transmitter selecting a set of beamforming weightsfrom the codebook to form by the antenna array a transmit/receive beampointing in a selected direction, wherein the receiver is configured toreceive from the transmitter one or more characteristic matrices and toconstruct the codebook using a second antenna array response matrix andthe one or more characteristic matrices received from the transmitter,and wherein the first antenna array response matrix contains, for aplurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix contains, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors.

According to another embodiment, a wireless communication network mayhave: an inventive transmitter as mentioned above; and one or moreinventive receivers as mentioned above.

According to another embodiment, a method may have the step of:determining a codebook used by a precoder of a transmitter, thetransmitter including an antenna array having a plurality of antennasfor a wireless communication with one or more receivers, and theprecoder connected to the antenna array, the codebook including aplurality of sets of beamforming weights for a plurality of directions,the beamforming weights in the codebook based on a first antenna arrayresponse matrix, wherein determining a codebook includes obtaining thefirst antenna array response matrix using a second antenna arrayresponse matrix and one or more characteristic matrices, and wherein thefirst antenna array response matrix contains, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix contains, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from the antennaarray, and the one or more characteristic matrices describing one ormore characteristics of the antenna array, and wherein the first antennaarray response matrix is modeled using the second antenna array responsematrix and the one or more characteristic matrices, each of the firstarray response vectors being a product of a characteristic matrix and acorresponding second array response vectors.

According to still another embodiment, a method may have the steps of:receiving and processing, at a receiver having an antenna for a wirelesscommunication with a transmitter, a radio signal received at the antennavia a radio channel, the radio signal including one or morecharacteristic matrices; and constructing a codebook at the receiver,the codebook including a plurality of sets of beamforming weights for aplurality of directions, the beamforming weights in the codebook beingbased on a first antenna array response matrix of an antenna array ofthe transmitter, and the transmitter selecting a set of beamformingweights from the codebook to form by an antenna array a transmit/receivebeam pointing in a selected direction, wherein the codebook isconstructed using a second antenna array response matrix and the one ormore characteristic matrices received from the transmitter, wherein thefirst antenna array response matrix contains, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix contains, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors.

Another embodiment may have a non-transitory digital storage mediumhaving stored thereon a computer program for performing a method havingthe step of: determining a codebook used by a precoder of a transmitter,the transmitter including an antenna array having a plurality ofantennas for a wireless communication with one or more receivers, andthe precoder connected to the antenna array, the codebook including aplurality of sets of beamforming weights for a plurality of directions,the beamforming weights in the codebook based on a first antenna arrayresponse matrix, wherein determining a codebook includes obtaining thefirst antenna array response matrix using a second antenna arrayresponse matrix and one or more characteristic matrices, and wherein thefirst antenna array response matrix contains, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix contains, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from the antennaarray, and the one or more characteristic matrices describing one ormore characteristics of the antenna array, and wherein the first antennaarray response matrix is modeled using the second antenna array responsematrix and the one or more characteristic matrices, each of the firstarray response vectors being a product of a characteristic matrix and acorresponding second array response vectors, when said computer programis run by a computer.

Another embodiment may have a non-transitory digital storage mediumhaving stored thereon a computer program for performing a method havingthe steps of: receiving and processing, at a receiver having an antennafor a wireless communication with a transmitter, a radio signal receivedat the antenna via a radio channel, the radio signal including one ormore characteristic matrices; and constructing a codebook at thereceiver, the codebook including a plurality of sets of beamformingweights for a plurality of directions, the beamforming weights in thecodebook being based on a first antenna array response matrix of anantenna array of the transmitter, and the transmitter selecting a set ofbeamforming weights from the codebook to form by an antenna array atransmit/receive beam pointing in a selected direction, wherein thecodebook is constructed using a second antenna array response matrix andthe one or more characteristic matrices received from the transmitter,wherein the first antenna array response matrix contains, for aplurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix contains, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors, when said computer program is run by a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are now described in further detailwith reference to the accompanying drawings, in which:

FIG. 1 shows a schematic representation of a system for implementingdifferent services using the concept of network slices;

FIG. 2 shows a block-based model of a MIMO communication system usingimplicit CSI feedback;

FIG. 3 shows an example of a (N_(Tx) ^(H), N_(Tx) ^(V), 2) uniformplanar array in FD-MIMO and the corresponding precoding matrixstructure;

FIG. 4 shows a block diagram of a MIMO communication system similar tothe system in FIG. 2 representing the transmission of controlinformation, CSI feedback and the construction of a codebook adapted toa real antenna response in accordance with embodiments of the presentinvention;

FIG. 5 shows a flowchart of a method in accordance with embodiments ofthe present invention of the control information transmission,generation of the codebook at the receiver and the feedback of CSIparameters; and

FIG. 6 illustrates an example of a computer system on which units ormodules as well as the steps of the methods described in accordance withthe inventive approach may execute.

DETAILED DESCRIPTION OF THE INVENTION

In the following, embodiments of the present invention are described infurther detail with reference to the enclosed drawings in which elementshaving the same or similar function are referenced by the same referencesigns.

In accordance with embodiments of the present invention in a wirelesscommunication system an antenna array of a transmitter is controlled bya codebook that has been established for a specific antenna array thatis different from the transmitter's antenna array. The transmitter'santenna array may be an antenna array having an arbitrary configuration,e.g. the transmitter's antenna array may include a 1-dimensional antennaarray, or a 2-dimensional antenna array, or a 3-dimensional antennaarray of any array configuration. The specific codebook, e.g. a 2D DFTbased codebook, is adapted to the actual or real antenna array of thetransmitter by one or more characteristic matrices describing theconfiguration or nature of the transmitter's antenna array. Thus, inaccordance with the inventive approach a specific codebook for oneantenna array, which is different from the antenna array of thetransmitter, is adapted for controlling the transmitter's antenna arrayso as to obtain the desired directional beam patterns.

In accordance with embodiments, a transmitter, e.g. a base stationserving one or more UEs or a UE communicating with a base station, isprovided including an antenna array having a plurality of antennas for awireless communication with one or more receivers and a precoderconnected to the antenna array, the precoder to apply a set ofbeamforming weights to the antenna array, the set of beamforming weightsselected from a codebook to form by the antenna array one or moretransmit/receive beams or nulls pointing in selected directions. Thecodebook includes a plurality of sets of beamforming weights for aplurality of directions. The beamforming weights in the codebook arebased on a first antenna array response matrix, the first antenna arrayresponse matrix defined by a second antenna array response matrix andone or more characteristic matrices. The first antenna array responsematrix contains, for a plurality of directions, first array responsevectors of the antenna array, the second antenna array response matrixcontains, for one or more of the plurality of directions, second arrayresponse vectors of another antenna array, the other antenna array beingdifferent from the antenna array, and the one or more characteristicmatrices describing one or more characteristics of the antenna array.

In the present invention, when referring to one or more of the pluralityof directions, this means one of the plurality of directions, two ormore of the plurality of directions, or all of the plurality ofdirections.

In other words, according the present invention, the array responsematrix of the transmitter's antenna array, which may have an arbitraryconfiguration, is based on a known array response matrix of anotherantenna array (different from the transmitter's antenna array) that isadapted to the actual or real world implementation of the transmitter'santenna array. This yields the array response for the transmitter'santenna array, which forms the basis for the precoder to be used by thetransmitter. The adapting uses the one or more characteristic matrices,which describe one or more of the characteristics of the antenna arrayof the transmitter, e.g. on the basis of a model of the transmitter'santenna array, a measurement of the transmitter's antenna array or acombination thereof.

In accordance with specific embodiments, a 2D DFT based codebook is usedas a basis for the codebook for the transmitter's antenna array. Thetransmitter's antenna array may be an arbitrary 3D shaped antenna array,e.g., a stacked uniform circular array. In order to achieve an accuratebeamforming and/or null steering for a realistic arbitrary antenna arrayat the transmitter, the codebook may be adapted to a real or measuredantenna array response of the transmitter, e.g., to the real or measuredfull-polarimetric 3D antenna array response of the transmitter.

The one or more characteristic matrices may be based on a model of theantenna array or on a measured antenna array response matrix containing,for some or all of the plurality of directions, measured array responsevectors determined from a measurement of the antenna array. Inaccordance with embodiments, the model of the antenna array or themeasurement of the antenna array may consider a directional andnon-identical nature of one or more of the respective antennas in theantenna array, and/or an electromagnetic coupling between the respectiveantennas in the antenna array, and/or other practical imperfections inthe antenna array. In accordance with further embodiments, the model ofthe antenna array or the measurement of the antenna array may describeone or more radiation characteristics of one or more of the antennas ofthe antenna array, the radiation characteristics including one or moreof a phase and an amplitude due to a position of an antenna in theantenna array, and/or a coupling imbalance, and/or a manufacturingimbalance.

In accordance with further embodiments, the codebook may be obtainedfrom a first codebook modified or adapted using the one or morecharacteristic matrices. The first codebook is defined by the secondarray response vectors of the second antenna array response matrix. Anadvantage of using the one or more characteristic matrices for adaptingthe codebook is that the information about the real antenna array at thetransmitter may be provided to a receiver, e.g. a UE served by the basestation, easily and without the need to transmit a huge amount ofinformation to the UE. In accordance with embodiments, by means of theone or more characteristic matrices the information of the realimplementation of the antenna array is transmitted by the base stationto the UE or may be downloaded by the UE. The availability of the actualeNodeB's array characteristics at the UE helps in beam refining, channelestimation and CSI feedback. The accuracy and amount of the CSIfeedback, and the accuracy of the channel estimation process heavilydepend on the availability of the actual codebook (or the arrayresponse) of the eNodeB at the UE. Transmitting the adapted arrayresponse to the UE as a whole would result in transmitting a huge amountof control information. In accordance with embodiments of the invention,only characteristic matrix is transmitted, thereby reducing the controlinformation overhead. In accordance with embodiments, in addition to thetransmission of the characteristic matrices also design parameters ofthe codebook for the other antenna array, like the 2D DFT matrix, may betransmitted to the receiver. The receiver may then generate the neededcodebook, e.g., a polarimetric 2D-DFT based codebook, defined by thereceived parameters and the received characteristic matrices. Thereceiver may then select the best codebook vectors from the generatedcodebook, and feedback the selected indices of the codebooks or theselected beamforming weights to the transmitter.

Thus, embodiments provide a receiver, e.g., a UE communicating with abase station or a base station serving one or more UEs, including anantenna for a wireless communication with a transmitter, and a signalprocessor to receive and process a radio signal received at the antennavia a radio channel. The receiver is configured to construct a codebookincluding a plurality of sets of beamforming weights for a plurality ofdirections, the beamforming weights in the codebook being based on afirst antenna array response matrix of an antenna array of thetransmitter, and the transmitter selecting a set of beamforming weightsfrom the codebook to form by the antenna array a transmit/receive beampointing in a selected direction. The receiver is configured toconstruct the codebook using a second antenna array response matrix andone or more characteristic matrices received from the transmitter. Thefirst antenna array response matrix contains, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix contains, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray.

In the following, the inventive approach will be described on the basisof embodiments of a DL communication in which the transmitter is thebase station or eNodeB having an arbitrary 3D antenna array, and thereceiver is a UE served by the eNodeB. First, embodiments using anon-polarimetric codebook will be described, followed by the descriptionof embodiments using a polarimetric codebook.

Non-Polarimetric Codebook

The response of an arbitrary 3D antenna array having N_(Tx) antennaports may be defined by the matrix A(φ,θ)ϵ

^(N) ^(Tx) ^(×MN). The matrix, also referred to as antenna arrayresponse matrix,A(φ,θ)=[a((φ₁,θ₁)a(φ₁,θ₂) . . . a(φ_(m),θ_(n)) . . . a(φ_(M),θ_(N-1))aφ_(M),θ_(N))]  (7)includes MN steering vectors a(φ_(m),θ_(n)), also to as antenna arrayresponse vectors. The steering vectors a(φ_(m),θ_(n)) include theresponses from all antenna ports for the m^(th) azimuth and n^(th)elevation directiona(φ_(m),θ_(n))=[a ₁(φ_(m),θ_(n))a ₂(φ_(m),θ_(n)) . . . a _(N) _(Tx)(φ_(m),θ_(n))]^(T)ϵ

^(N) ^(Tx) ^(×1),   (8)∀m=1,2, . . . ,M and n=1,2, . . . ,N.

The parameters M and N denote the number of samples in the azimuth rangeφ_(m)ϵ[{tilde over (φ)}₀, {tilde over (φ)}₁) and elevation rangeθ_(n)ϵ[{tilde over (θ)}₀, {tilde over (θ)}₁), respectively.

Step 1: Each steering vector a(φ_(m), θ_(n)) of the arbitrary 3D antennaarray may be written as a product of a characteristic matrix G of theantenna array and a steering vector d(φ_(m), θ_(n)),a(φ_(m),θ_(n))=Gd(φ_(m),θ_(n))  (9)If the matrix Gϵ

^(N) ^(Tx) ^(×Q) corresponds to the characteristic matrix of the arrayresponse A, the steering vector d(φ_(m), θ_(n)) is given as a Kroneckerproduct of two Vandermonde vectors d(φ_(m)) and d(θ_(n)),

$\begin{matrix}{\mspace{79mu}{{d\left( {\varphi_{m},\theta_{n}} \right)} = {{{d\left( \varphi_{m} \right)} \otimes {d\left( \theta_{n} \right)}} \in {{\mathbb{C}}^{Q \times 1}\mspace{14mu}{where}}}}} & (10) \\{{d\left( \varphi_{m} \right)} = {\left\lbrack {e^{{- \frac{j{({Q_{1} - 1})}}{2}}\varphi_{m}},\ldots\mspace{14mu},e^{{- j}\;\varphi_{m}},1,e^{j\;\varphi_{m}},\ldots\mspace{14mu},e^{\frac{j{({Q_{1} - 1})}}{2}\varphi_{m}}} \right\rbrack^{T} \in {\mathbb{C}}^{Q_{1} \times 1}}} & (11) \\{{d\left( \theta_{n} \right)} = {\left\lbrack {e^{{- \frac{j{({Q_{2} - 1})}}{2}}\theta_{n}},\ldots\mspace{14mu},e^{{- j}\;\theta_{n}},1,e^{j\;\theta_{n}},\ldots\mspace{14mu},e^{\frac{j{({Q_{2} - 1})}}{2}\theta_{n}}} \right\rbrack^{T} \in {\mathbb{C}}^{Q_{2} \times 1}}} & (12)\end{matrix}$such that Q₁·Q₂=Q.

By collecting MN vectors of size Q×1 defined in equation (10), thematrix D(φ_(m), θ_(n)) may be defined asD(φ,θ)=[d(φ₁,θ₁)d(φ₁,θ₂) . . . d(φ_(m),θ_(n)) . . .d(φ_(m),θ_(N-1))d(φ_(M),θ_(N))]ϵ

^(Q×MN)  (13)Q₁ and Q₂ denote the number of antenna ports in the horizontal (row) andvertical (columns) directions, respectively.

The N_(Tx)×Q sized non-square characteristic matrix G provide amathematical model of the transmitter's antenna array and contain asufficiently accurate descriptions of the antenna element radiationcharacteristics including phases and amplitudes due to elementpositions, coupling and manufacturing imbalances.

The characteristic matrix G may be computed by solving the followingoptimization problem

$\begin{matrix}{G_{opt} = {\underset{G \in {\mathbb{C}}^{{N_{Tx} \cdot} \times Q}}{argmin}{{A - {GD}}}_{F}^{2}}} & (14)\end{matrix}$

One potential solution to (14) is given by the concept of the EffectiveAperture Distribution Function (EADF) of the antenna array. The EADFrepresents the inverse Fourier transform of the array response over theazimuth range φ_(m)ϵ[{tilde over (φ)}₀, {tilde over (φ)}₁) and elevationrange θ_(n)ϵ[{tilde over (θ)}₀, {tilde over (θ)}₁). To calculate theEADF matrix G_(EADF), the array response has to be available as discreteperiodic functions with a period 2π. For the q-th row of A, the elementg_(iq) of matrix G is given by

$\begin{matrix}{g_{iq} = {\frac{1}{R}{\sum\limits_{r = 1}^{R}\;{a_{ir}e^{\frac{j\; 2\;\pi}{R}{({r - 1})}{({q - 1})}}}}}} & (15)\end{matrix}$for 1≤q≤Q and i=1, . . . , N_(Tx) with R=MN.

Alternatively, the characteristic matrix G may be computed by the leastsquares (LS) solution given byG _(LS) =AD ^(H)(DD ^(H))⁻¹  (16)

The 2D DFT steering matrix D(φ_(m), θ_(n)) may also be mapped to thearray response of a “virtual” ideal URA having Q “virtual” antenna portsby introducing the angle transformations α=sin(φ_(m)) and β=cos(θ_(n))on the Vandermonde vector shown in equations (11) and (12). Here α and βrepresent the phases of the rectangular array in horizontal and verticaldirections, respectively.

In contrast to an array interpolation as described in reference [5], ora beamspace transformation as described in reference [6], thetransformation approach in equation (9) does not interpolate thesteering vectors from the real array response to another array response,rather it changes the mathematical model of the real array response.

The parameter Q, specifying the dimension of the vectors in the 2D DFTmatrix is a design parameter and may be chosen freely such thatN_(Tx)<<Q. The larger Q, the smaller is the approximation or fittingerrors O(Q) of the transformation of the array responsesO(Q)=∥A−GD∥ _(F) ²  (17)

Using more samples Q to approximate the characteristic matricesincreases the sizes.

At the transmitter, the characteristic matrix G and the 2D-DFT matrix Dmay be used to design a 2D-DFT based codebook that is adapted to the 3Dantenna array response at the transmitter.Ω={Gd(φ_(m),θ_(n))|∀(m,n)ϵ{1≤m≤M,1≤n≤N}}  (18)

Step 2: The characteristic matrix G and design parameters of the 2D DFTmatrix (Q, M, N) may be transmitted to the receiver side. The receivermay generate a 2D-DFT based codebook Ω, defined by the parameters (Q, M,N) and the characteristic matrix G. The receiver may select the bestcodebook vectors from the codebook, and feedback the selected indices ofthe codebook or the codebook entries (beamforming weights) to thetransmitter (e.g. eNodeB).

Polarimetric Codebook

In the embodiments described so far, no assumptions have been made withrespect to the array geometry or the number of polarizations involved inthe array, and the codebook and antenna response considerationsdescribed so far are non-polarimetric, i.e., the discussion of the arrayresponse or codebook was not discussed in the context of sensitivity ofthe antenna array to the polarization state of the transmitted orreceived wavefront. However, the present invention is not limited tosuch embodiments.

Although some codebook designs may be non-polarimetric, and, therefore,tend to ignore the practical aspects of the array in the design, theinventive approach in accordance with further embodiments considers thepolarization-based response of the antennas as otherwise the beamformingefforts may turn out to be totally useless (see references [7] and [8]).Thus, in accordance with embodiments the responses of the antenna portsof the antenna array to the horizontal and vertical polarization of thetransmitted/received wavefront are considered.

The antennas in the array may be oriented in different directions to besensitive to wavefronts with varying polarizations. Ideally, an antennaoriented along a particular direction should respond to a wavefrontpolarized only along a certain direction and reject all the componentspolarized in any other direction. In the literature, the direction oforientation of the antennas is the same as the polarization angle theantenna responds to and, therefore, the term ‘polarization of theantenna’ is used in place of ‘orientation of the antenna, however,herein the term ‘orientation’ is used when referring to antennas toavoid any confusion with the wave polarization. In practice, however,due to the non-ideality of the antennas, the antennas tend totransmit/receive components of the wavefront across multiplepolarizations. Therefore, in accordance with embodiments, the responseof the antennas in the array across two orthogonal polarizations of thewavefront, horizontal and vertical, is considered for the polarimetriccodebook design. The terms ‘horizontal and vertical’ mentioned hereinare the polarizations of the wavefront, and not the horizontal andvertical orientations/polarizations of the antennas (or the horizontaland vertical arrangement of the antennas in a uniform planar array inFD-MIMO, for example). There is also a specific notation used to denotethe response of the antenna to a particular wave polarization: thesubscript/superscript ‘(h)’ in a matrix or vector denotes theassociation with the horizontal polarization of the wavefront and thesubscript/superscript ‘(v)’ denotes the association with the verticalpolarization of the wavefront.

The response of an arbitrary 3D antenna array having N_(Tx) antennaports is defined by two matrices A_((h))(φ, θ)ϵ

^(N) ^(Tx) ^(×MN) and A_((v))(φ, θ)ϵ

^(N) ^(Tx) ^(×MN) with respect to the horizontal and verticalpolarization of the transmitted/received wavefront, respectively. Thematrices, also referred to as antenna array response matrices,A _((h))(φ,θ)=[a _((h))(φ₁,θ₁)a _((h))(φ₁,θ₂) . . . a_((h))(φ_(m),θ_(n)) . . . a _((h))(φ_(M),θ_(N-1))a_((h))(φ_(M),θ_(N))]  (19)A _((v))(φ,θ)=[a _((v))(φ₁,θ₁)a _((v))(φ₁,θ₂) . . . a_((v))(φ_(m),θ_(n)) . . . a _((v))(φ_(M),θ_(N-1))a_((v))(φ_(M),θ_(N))]  (20)include MN steering vectors a_((h))(φ_(m), θ_(n)) and a_((v))(φ_(m),θ_(n)) for the horizontal and vertical polarizations, respectively. Thesteering vectors are also referred to as antenna array response vectors.The steering vectors a_((h))(φ_(m), θ_(n)) and a_((v))(φ_(m), θ_(n))include the responses from all antenna ports for the m^(th) azimuth andn^(th) elevation directiona _((h))(φ_(m),θ_(n))[a _((h)1)(φ_(m),θ_(n))a _((h)2)(φ_(m),θ_(n)) . . .a _((h)N) _(Tx) (φ_(m),θ_(n))]^(T)ϵ

^(N) ^(Tx) ^(×1)  (21)a _((v))(φ_(m),θ_(n))[a _((v)1)(φ_(m),θ_(n))a _((v)2)(φ_(m),θ_(n)) . . .a _((v)N) _(Tx) (φ_(m),θ_(n))]^(T)ϵ

^(N) ^(Tx) ^(×1)  (22)∀m=1,2, . . . ,M and n=1,2, . . . ,N.

The parameters M and N denote the number of samples in the azimuth rangeφ_(m) ϵ[{tilde over (φ)}₀, {tilde over (φ)}₁) and elevation rangeθ_(n)ϵ[{tilde over (θ)}₀, {tilde over (θ)}₁), respectively.

Step 1: Each steering vector a_((h))(φ_(m),θ_(n)) with respect to thehorizontal polarization of the arbitrary 3D antenna array of thetransmitter may be written as a product of a characteristic matrixG_((h)) of the antenna array and a steering vector d(φ_(m), θ_(n))a _((h))(φ_(m),θ_(n))=G _((h)) d(φ_(m),θ_(n))  (23)and, similar, each steering vector a_((v))(φ_(m), θ_(n)) with respect tothe vertical polarization may be written asa _((v))(φ_(m),θ_(n))=G _((v)) d(φ_(m),θ_(n)).  (24)

If the matrix G_((h))ϵ

^(N) ^(Tx) ^(×Q) corresponds to the characteristic matrix of the arrayresponse A_((h)), the steering vector d(φ_(m), θ_(n)) is given as aKronecker product of two Vandermonde vectors d(φ_(m)) and d(θ_(n)),

$\begin{matrix}{\mspace{79mu}{{d\left( {\varphi_{m},\theta_{n}} \right)} = {{{d\left( \varphi_{m} \right)} \otimes {d\left( \theta_{n} \right)}} \in {{\mathbb{C}}^{Q \times 1}\mspace{14mu}{where}}}}} & (25) \\{{d\left( \varphi_{m} \right)} = {\left\lbrack {e^{{- \frac{j{({Q_{1} - 1})}}{2}}\varphi_{m}},\ldots\mspace{14mu},e^{{- j}\;\varphi_{m}},1,e^{j\;\varphi_{m}},\ldots\mspace{14mu},e^{\frac{j{({Q_{1} - 1})}}{2}\varphi_{m}}} \right\rbrack^{T} \in {\mathbb{C}}^{Q_{1} \times 1}}} & (26) \\{{d\left( \theta_{n} \right)} = {\left\lbrack {e^{{- \frac{j{({Q_{2} - 1})}}{2}}\theta_{n}},\ldots\mspace{14mu},e^{{- j}\;\theta_{n}},1,e^{j\;\theta_{n}},\ldots\mspace{14mu},e^{\frac{j{({Q_{2} - 1})}}{2}\theta_{n}}} \right\rbrack^{T} \in {\mathbb{C}}^{Q_{2} \times 1}}} & (27)\end{matrix}$such that Q₁·Q₂=Q.

By collecting MN vectors of size Q×1 defined in equation (25), thematrix D(φ_(m), θ_(n)) may be defined asD(φ,θ)=[d(φ₁,θ₁)d(φ₁,θ₂) . . .d(φ_(m),θ_(n))d(φ_(M),θ_(N-1))d(φ_(M),θ_(N))]ϵ

^(Q×MN)  (28)

Q₁ and Q₂ denote the number of antenna ports in the horizontal (row) andvertical (columns) directions, respectively.

The N_(Tx)×Q sized non-square characteristic matrices G_((h)) andG_((v)) provide a mathematical model of the antenna array and contain asufficiently accurate descriptions of the antenna element radiationcharacteristics including phases and amplitudes due to elementpositions, coupling and manufacturing imbalances with respect to the twopolarizations.

The characteristic matrices G_((h)) and G_((v)) may be computed bysolving the following optimization problem

$\begin{matrix}{{{\overset{\_}{G}}_{opt} = {\underset{\overset{\_}{G} \in {\mathbb{C}}^{{N_{Tx} \cdot 2} \times Q}}{argmin}{{\overset{\_}{A} - {\overset{\_}{G}D}}}_{F}^{2}}}\mspace{14mu}{{{{where}\mspace{14mu}\overset{\_}{A}} = {\begin{bmatrix}A_{(h)} \\A_{(v)}\end{bmatrix} \in {\mathbb{C}}^{{N_{Tx} \cdot 2} \times {MN}}}},{\overset{\_}{G} = {\begin{bmatrix}G_{(h)} \\G_{(v)}\end{bmatrix} \in {\mathbb{C}}^{{N_{Tx} \cdot 2} \times Q}}}}} & (29)\end{matrix}$and G _(opt) is the solution to the optimization problem. One potentialsolution to equation (29) is given by the concept of the EffectiveAperture Distribution Function (EADF) of the antenna array. The EADFrepresents the inverse Fourier transform of the array response over theazimuth range φ_(m)ϵ[{tilde over (φ)}₀,{tilde over (φ)}₁) and elevationrange θ_(n)ϵ[{tilde over (θ)}₀, {tilde over (θ)}₁). To calculate theEADF matrices G_((h)EADF) and G_((v)EADF), the array response has to beavailable as discrete periodic functions with a period 2π in the azimuthand the elevation, respectively. For the q-th row of A_((h)), theelement g_((h)iq) of matrix G_((h)) is given by

$\begin{matrix}{g_{{(h)}{iq}} = {\frac{1}{R}{\sum\limits_{r = 1}^{R}\;{a_{{(h)}{ir}}e^{\frac{j\; 2\;\pi}{R}{({r - 1})}{({q - 1})}}}}}} & (30)\end{matrix}$for 1≤q≤Q and i=1, . . . , N_(Tx) with R=MN.

Similarly, the element g_((v)iq) of matrix G_((v)) is given by

$\begin{matrix}{g_{{(v)}{iq}} = {\frac{1}{R}{\sum\limits_{r = 1}^{R}\;{a_{{(v)}{ir}}e^{\frac{j\; 2\;\pi}{R}{({r - 1})}{({q - 1})}}}}}} & (31)\end{matrix}$

Alternatively, the characteristic matrices G_((h)) and G_((v)) may alsobe computed by the least squares (LS) solution given by

$\begin{matrix}{{{\overset{\_}{G}}_{LS} = {{\overset{\_}{A}{D^{H}\left( {DD}^{H} \right)}^{- 1}\mspace{14mu}{where}\mspace{14mu}{\overset{\_}{G}}_{LS}} = {\begin{bmatrix}G_{{(h)}{LS}} \\G_{{(v)}{LS}}\end{bmatrix} \in {\mathbb{C}}^{{N_{Tx} \cdot 2} \times Q}}}},} & (32)\end{matrix}$such that Q<MN for the inverse to exist.

It is noted that the solutions obtained by the EADF in equations (30)and (31) and by the LS approach in equation (32) may be identical whenthe angles in equations (21) and (22) are chosen such that D correspondsto a 2D DFT matrix.

Using the transformation approach in equations (23) and (24), the arrayresponses A_(h)(φ_(m), θ_(n)) and A_(v)(φ_(m), θ_(n)) are transformed toa new array response D(φ_(m), θ_(n)) that is given by a 2D DFT matrix.

The new 2D DFT steering matrix D(φ_(m), θ_(n)) may also be mapped to thearray response of a “virtual” ideal URA having Q “virtual” antenna portsby introducing angle transformations α=sin(φ_(m)) and β=cos(θ_(n)) onthe Vandermonde vectors shown in equations (26) and (27). α and βrepresent the phases of the rectangular array in horizontal and verticaldirections, respectively.

In contrast to the array interpolation described in reference [5], or tothe beamspace transformation described in reference [6], thetransformation approach in equations (23) and (24) does not interpolatethe steering vectors from the real array response to another arrayresponse, rather it changes the mathematical model of the real arrayresponse.

The parameter Q, specifying the dimension of the vectors in the 2D DFTmatrix is a design parameter and may be chosen freely such thatN_(Tx)<<Q. The larger Q, the smaller is the approximation or fittingerrors O_((h)) (Q) or O_((v))(Q) of the transformation of the arrayresponsesO _((h))(Q)=∥A _((h)) −G _((h)) D∥ _(F) ²  (33)O _((v))(Q)=∥A _((v)) −G _((v)) D∥ _(F) ²  (34)

Using more samples Q to approximate the characteristic matrices mayincrease the sizes.

Design of polarimetric codebook (Ω_((h)), Ω_((v)))

The characteristic matrices G_((h)) and G_((v)) and the 2D-DFT matrix Dmay be used to design a 2D-DFT based codebook that is adapted to thepolarimetric 3D antenna array response at the transmitter. Thepolarimetric codebook includes the two sub-codebooks Ω_((h)) and Ω_((v))with respect to the horizontal and vertical polarization, respectivelyΩ_((h)) ={G _((h)) d(φ_(m),θ_(n))|∀(m,n)ϵ{1≤m≤M,1≤n≤N}}  (35)Ω_((v)) ={G _((v)) d(φ_(m),θ_(n))|∀(m,n)ϵ{1≤m≤M,1≤n≤N}}  (36)where, the sub-codebooks Ω_((h)) and Ω_((v)) are defined over a total ofMN points in the angular range for M points in the azimuth and N pointsin the elevation with a resolution of

$\frac{\varphi_{1} - \varphi_{0}}{M}$in the azimuth and

$\frac{\theta_{1} - \theta_{0}}{N}$the elevation.

The corresponding precoders used at the transmitter are then based onsubsets of vectors of Ω_((h)) and Ω_((v)).

Additionally, the entries in equations (35) and (36) may be normalizedto satisfy a power constraint at the transmitter. For example, one wayto constrain the power is to multiply the codebook entries in Ω_((h))and Ω_((v)) by the normalization matrices N_((h)) and N_((v)),respectively. The normalized codebooks satisfying a specific powerconstraint are then given byΩ_((h)) =G _((h)) d(φ_(m),θ_(n))N _((h))  (37)Ω_((v)) =G _((v)) d(φ_(m),θ_(n))N _((v))  (38)where N_((h)) and N_((v)) are diagonal matrices of size MN×MN with theentries being defined such that a specific power constraint issatisfied.

Step 2: The characteristic matrices G_((h)) and G_((v)) and the designparameters of the 2D DFT matrix (Q, M, N) may be transmitted to thereceiver side. The receiver may generate a polarimetric 2D-DFT basedcodebook Ω_((h)) and Ω_((v)), defined by the parameters (Q, M, N) andthe characteristic matrices G_((h)) and G_((v)). The receiver may selectthe best codebook vectors from Ω_((h)) and Ω_((v)), and feedback theselected indices of the codebooks or the selected beamforming weights tothe transmitter (e.g. eNodeB).

The characteristic matrix(ces) may be determined by the eNodeB offline,for example the computations may be performed while there is nocommunication with the UE, and then the characteristic matrix(ces) maybe downloaded by the UE to construct the codebook of the eNodeB that isadapted to the ‘non-ideal’ array response. The characteristic matricessent to the UE are a part of the ‘control information’ as shown in FIG.4.

FIG. 4 shows a block diagram of a MIMO system showing an embodiment ofthe transmission of control information, CSI feedback and theconstruction of a codebook adapted to the real antenna response of thetransmitter's antenna array. The embodiment of FIG. 4 is described withreference to the use of a polarimetric codebook, however, the presentinvention is not limited to such an embodiment. The inventive approachis equally applicable to non-polarimetric codebooks. Those elements ofthe MIMO system corresponding to those already described above withreference to FIG. 2 have assigned thereto the same reference signs. Inaccordance with embodiments of the present invention, the base station200 includes the characteristic matrices G. The characteristic matricesG may be determined as described above and may be stored at the basestation 200, e.g., in a storage device 210. On the basis of thecharacteristic matrices G and the codebook 208 for a certain antennaarray (different from the antenna array 202), which may be a 2D DFTcodebook for URA antenna array, the array response adapted codebook 212is obtained and used by the precoder to obtain an expected directionalbeam pattern despite. In accordance with embodiments, characteristicmatrices G may be transmitted to the UE 300, e.g., as controlinformation 504 in the PDCCH 502 of the wireless communication system.The control information 504 sent to the receiver 300 may include thecharacteristic matrices G_((h)) and G_((v)) and the design parameters(Q, M, N) for the codebook 208. Using these parameters, the receivergenerates at 306 the 2D DFT matrix D representing the codebook 208, andconstructs at 308 the polarimetric codebooks Ω_((h)) and Ω_((v)) thatare adapted to the 3D antenna response of the array 202 used at thetransmitter 200.

The UE 300, on the basis of the constructed codebooks of the eNodeB orbase station 200, performs an improved beam refining, channel estimationand CSI feedback taking into account the real antenna array at thetransmitter 200, thereby improving the accuracy of the channelestimation process, as well as the accuracy and amount of the CSIfeedback 500.

FIG. 5 is a flowchart of a method in accordance with embodiments of thepresent invention of the control information transmission, generation ofthe codebook at the receiver and the feedback of CSI parameters. AtS100, the characteristic matrices {G_((h)), G_((v))} and the 2D DFTmatrix D of size Q×MN are computed based on the 3D antenna response ofthe transmitter 200, and, at S102, the characteristic matrices {G_((h)),G_((v))} and the design parameters (Q, M, N) are transmitted as thecontrol information 504 to the receiver 300. At S104, the 2D DFTcodebook is generated at the receiver 300 based on (Q, M, N) and thepolarimetric codebooks {Ω_((h)),Ω₀,)} are constructed at the receiver300. At S106 the CSI parameters (CQI, PMI, RI) are computed and fed backfrom the receiver 300 to the transmitter 200. At S108, the precoders areselected from the polarimetric codebooks {Ω_((h)),Ω_((v))} for the nexttransmission based on the received feedback from the receiver 300.

FURTHER EMBODIMENTS 1. Accuracy and Null/Beam Forming Capabilities

The array manifold technique offers the flexibility in choosing thedimensions/size of the non-squared characteristic matrices G_((h)) andG_((v)), depending on the desired accuracy in the angular domain.Choosing more samples in the characteristic matrices achieves anaccurate beamforming as well as null steering in a desired direction. Onthe other hand, a small reduction of the number of samples in the EADFmatrices may not drastically reduce the beamforming capability andaccuracy, but it may have a large effect on the desired null steeringcapability.

2. Reduction of Control Information Overhead by Compression of MatricesG_((h)) and G_((v))

Even though the sizes of the characteristic matrices are small comparedto the 3D antenna response matrices A_((h))(φ, θ) and A_((v))(φ, θ),their sizes may be further reduced. One approach is to choose areference row (e.g., an EADF of one of the available antenna ports) andtake the residual of each row (all other antenna ports) with respect tothe reference row chosen. By doing so, many values in the residual areclose to zero. By ignoring the terms close to zero and sending only thesignificant entries along with the reference row chosen, the overhead inthe control information may be reduced. The receiver may reconstruct thecharacteristic matrices.

3. Using the Characteristic Matrices as a Fixed Precoder

If only a small number of samples from the EADF of the antenna elementsare used to represent the characteristic matrices, the characteristicmatrices may also be used as fixed precoders. By doing so, there is noneed to send these matrices to the receiver via the control channel. Asa consequence, the control information overhead may be reduced. Thefixed precoder at the transmitter may be defined as a function f(⋅) ofthe characteristic matrices G_((h)) and G_((v)). By doing so, thereceiver may simply use the 2D DFT based codebook and feedbacks the CSIparameters based on the effective channel H_(eff)=HF, where F=ƒ(G_((h)),G_((v))). Only the design parameters (Q, M, N) are used to construct thearray response at the receiver.

4. Application of Characteristic Matrices as a Precoder in the MultiuserScenario

In a multi-user scenario the capability of the characteristic matricesto steer nulls and beams in the desired directions may be exploited. Byappropriately choosing the precoders from the designed/constructedpolarimetric codebook, nulls may be steered in the desired directions ofeach user and hence the multi-user interference may be mitigated.

5. Alternative Embodiments

Although the embodiments above have been described for a downlink datatransmission in which the base station operates as a transmitter and theUE operates as a receiver, the inventive approach is not limited to suchembodiments. In accordance with other embodiments, the inventiveapproach is equally applicable in an uplink (UL) transmission of themobile multiple input multiple output communication system, e.g., thecommunication link carrying data traffic from the mobile user equipment(UE) to the base station (eNodeB). In such embodiments the UE operatesas the transmitter and the base station operates as the receiver.

Although the embodiments above have been described with reference to a1-dimensional antenna array, like a Uniform Linear Array, or a2-dimensional antenna array, like a Uniform Planar Array, the inventiveapproach is not limited to such embodiments. In accordance with otherembodiments, the inventive approach is equally applicable to any antennaarray, like a 1-dimensional antenna array, or a 2-dimensional antennaarray or a 3-dimensional antenna array of any array configuration.

Further embodiments of the present invention are now described.

A 1^(st) embodiment provides a transmitter, comprising an antenna arrayhaving a plurality of antennas for a wireless communication with one ormore receivers, and a precoder connected to the antenna array, theprecoder to apply a set of beamforming weights to the antenna array, theset of beamforming weights selected from a codebook to form by theantenna array one or more transmit/receive beams or nulls pointing inselected directions, wherein the codebook includes a plurality of setsof beamforming weights for a plurality of directions, wherein thebeamforming weights in the codebook are based on a first antenna arrayresponse matrix, the first antenna array response matrix defined by asecond antenna array response matrix and one or more characteristicmatrices, and wherein the first antenna array response matrix contains,for a plurality of directions, first array response vectors of theantenna array, the second antenna array response matrix contains, forone or more of the plurality of directions, second array responsevectors of another antenna array, the other antenna array beingdifferent from the antenna array, and the one or more characteristicmatrices describing one or more characteristics of the antenna array.

A 2^(nd) embodiment provides the transmitter of the 1^(st) embodiment,wherein the one or more characteristic matrices are based on a model ofthe antenna array or on a measured antenna array response matrixcontaining, for some or all of the plurality of directions, measuredarray response vectors determined from a measurement of the antennaarray.

A 3^(rd) embodiment provides the transmitter of the 2^(nd) embodiment,wherein the model of the antenna array or the measurement of the antennaarray considers a directional and non-identical nature of one or more ofthe respective antennas in the antenna array, and/or an electromagneticcoupling between the respective antennas in the antenna array, and/orother practical imperfections in the antenna array.

A 4^(th) embodiment provides the transmitter of the 1^(st) or 2^(nd)embodiments, wherein the model of the antenna array or the measurementof the antenna array describes one or more radiation characteristics ofone or more of the antennas of the antenna array, the radiationcharacteristics including one or more of a phase and an amplitude due toa position of an antenna in the antenna array, and/or a couplingimbalance, and/or a manufacturing imbalance.

A 5^(th) embodiment provides the transmitter of one of the 1^(st) to4^(th) embodiments, wherein the other antenna array comprises a1-dimensional antenna array, or a 2-dimensional antenna array or a3-dimensional antenna array.

A 6^(th) embodiment provides the transmitter of one of the 1^(st) to5^(th) embodiments, wherein the second antenna array response matrix isdefined by a 2D-DFT based matrix.

A 7^(th) embodiment provides the transmitter of one of the 1^(st) to6^(th) embodiments, wherein the first antenna array response matrix ismodeled using the second antenna array response matrix and the one ormore characteristic matrices.

An 8^(th) embodiment provides the transmitter of one of the 1^(st) to rembodiments, wherein the codebook is obtained from a first codebookmodified using the one or more characteristic matrices, the firstcodebook defined by the second array response vectors of the secondantenna array response matrix.

A 9^(th) embodiment provides the transmitter of one of the 1^(st) to8^(th) embodiments, wherein the first antenna array response matrixcomprises a first antenna array response submatrix and a second antennaarray response submatrix, wherein the first antenna array responsesubmatrix contains, for the plurality of directions, the array responsevectors of all antenna ports in the antenna array with respect to thehorizontal polarization of an wavefront, and wherein the second antennaarray response submatrix contains, for the same directions as in thefirst submatrix, the array response vectors of all antenna ports in theantenna array with respect to the vertical polarization of thewavefront.

A 10^(th) embodiment provides the transmitter of the 9^(th) embodiment,comprising a first characteristic matrix corresponding to the horizontalpolarization, the first antenna array response submatrix modeled by thesecond array response vectors of the second antenna array responsematrix and the first characteristic matrix, and a second characteristicmatrix corresponding to the vertical polarization, the second antennaarray response submatrix modeled by the second array response vectors ofthe second antenna array response matrix and the second characteristicmatrix.

An 11^(th) embodiment provides the transmitter of the 10^(th)embodiment, wherein the codebook comprises a polarimetric codebook, thepolarimetric codebook containing a first sub-codebook and a secondsub-codebook, wherein the first sub-codebook contains beamformingweights for all antenna ports with respect to the horizontalpolarization, the beamforming weights based on the first antenna arrayresponse submatrix, and wherein the second sub-codebook containsbeamforming weights for all antenna ports with respect to the verticalpolarization, the beamforming weights based on the second antenna arrayresponse submatrix.

A 12^(th) embodiment provides the transmitter of one of the 1^(st) to11^(th) embodiments, wherein the transmitter is configured to transmitthe one or more characteristic matrices to the one or more receivers.

A 13^(th) embodiment provides the transmitter of the 12^(th) embodiment,wherein the transmitter is configured to transmit the one or morecharacteristic matrices as part of control information to the receiver.

A 14^(th) embodiment provides the transmitter of the 12^(th) or 13^(th)embodiment, wherein the transmitter is configured to transmit one ormore design parameters of the codebook of the other antenna array to thereceiver side.

A 15^(th) embodiment provides the transmitter of one of the 1^(st) to14^(th) embodiments, wherein the antenna array comprises a 1-dimensionalantenna array, or a 2-dimensional antenna array or a 3-dimensionalantenna array.

A 16^(th) embodiment provides the transmitter of the 15^(th)embodiments, wherein the 2-dimensional antenna array or the3-dimensional antenna array controls the radio wave for beamforming bothin the vertical direction and the horizontal direction, the precoderhaving a dual-stage structure, the first stage containing the sets ofbeamforming weights based on the codebook to be applied to each antennaelement of the antenna array, the second stage containing coefficientsthat select and/or linearly combine the beams defined by the first stageto obtain a desired beam pattern.

A 17^(th) embodiment provides a receiver, comprising an antenna for awireless communication with a transmitter, and a signal processor toreceive and process a radio signal received at the antenna via a radiochannel, wherein the receiver is configured to construct a codebookincluding a plurality of sets of beamforming weights for a plurality ofdirections, the beamforming weights in the codebook being based on afirst antenna array response matrix of an antenna array of thetransmitter, and the transmitter selecting a set of beamforming weightsfrom the codebook to form by the antenna array a transmit/receive beampointing in a selected direction, wherein the receiver is configured toconstruct the codebook using a second antenna array response matrix andone or more characteristic matrices received from the transmitter, andwherein the first antenna array response matrix contains, for aplurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix contains, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray.

An 18^(th) embodiment provides the receiver of the 17^(th) embodiment,wherein the receiver is configured for a wireless communication with atransmitter of one of the 1^(st) to 16^(th) embodiments.

A 19^(th) embodiment provides the receiver of the 17^(th) or 18^(th)embodiments, comprising a channel estimator to estimate a state of theradio channel, wherein the receiver is configured to determine, based onthe radio channel estimation and the constructed codebook, a feedback tothe transmitter, the transmitter selecting, on the basis of thefeedback, a set of beamforming weights from the codebook to form thetransmit/receive beam, and wherein the feedback includes control signalson the basis of which the transmitter selects a set of beamformingweights, or the feedback transmits a set of beamforming weights from thereceiver to the transmitter.

A 20^(th) embodiment provides a wireless communication network,comprising a transmitter of one of the 1^(st) to 16^(th) embodiments,and one or more receivers of one of the 17^(th) to 19^(th) embodiments.

A 21^(st) embodiment provides the wireless communication network of the20^(th) embodiment, comprising a cellular network, a wireless local areanetwork or a wireless sensor system.

A 22^(nd) embodiment provides the wireless communication network of the20^(th) or 21^(st) embodiments, wherein the transmitter comprises a basestation serving a user equipment, or a user equipment served by a basestation, or wherein the receiver comprises a base station serving a userequipment, or a user equipment served by a base station.

A 23^(rd) embodiment provides the wireless communication network of the22^(nd) embodiment, wherein the user equipment comprises a mobileterminal, or an IoT device, or a device implemented inside a movingvehicle, like a moving vehicle, e.g., a car or a robot, or inside aflying device, e.g., an unmanned aerial vehicle (UAV) or a plane.

A 24^(th) embodiment provides the wireless communication network of oneof the 20^(th) to 23^(rd) embodiments, using an IFFT (Inverse FastFourier Transform) based signal, wherein the IFFT based signal includesOFDM with CP, DFT-s-OFDM with CP, IFFT-based waveforms without CP,filtered OFDM (f-OFDM1, FBMC, GFDM or UFMC.

A 25^(th) embodiment provides a method, comprising determining acodebook used by a precoder of a transmitter, the transmitter includingan antenna array having a plurality of antennas for a wirelesscommunication with one or more receivers, and the precoder connected tothe antenna array, the codebook including a plurality of sets ofbeamforming weights for a plurality of directions, the beamformingweights in the codebook based on a first antenna array response matrix,wherein determining a codebook includes obtaining the first antennaarray response matrix using a second antenna array response matrix andone or more characteristic matrices, and wherein the first antenna arrayresponse matrix contains, for a plurality of directions, first arrayresponse vectors of the antenna array, the second antenna array responsematrix contains, for one or more of the plurality of directions, secondarray response vectors of another antenna array, the other antenna arraybeing different from the antenna array, and the one or morecharacteristic matrices describing one or more characteristics of theantenna array.

A 26^(th) embodiment provides the method of the 25^(th) embodiment,comprising transmitting the one or more characteristic matrices to theone or more receivers to allow the receiver to construct a codebook atthe receiver.

A 27^(th) embodiment provides a method, comprising receiving andprocessing, at a receiver having an antenna for a wireless communicationwith a transmitter, a radio signal received at the antenna via a radiochannel, the radio signal including one or more characteristic matrices,and constructing a codebook at the receiver, the codebook including aplurality of sets of beamforming weights for a plurality of directions,the beamforming weights in the codebook being based on a first antennaarray response matrix of an antenna array of the transmitter, and thetransmitter selecting a set of beamforming weights from the codebook toform by an antenna array a transmit/receive beam pointing in a selecteddirection, wherein the codebook is constructed using a second antennaarray response matrix and the one or more characteristic matricesreceived from the transmitter, and wherein the first antenna arrayresponse matrix contains, for a plurality of directions, first arrayresponse vectors of the antenna array, the second antenna array responsematrix contains, for one or more of the plurality of directions, secondarray response vectors of another antenna array, the other antenna arraybeing different from the transmitter's antenna array, and the one ormore characteristic matrices describing one or more characteristics ofthe transmitter's antenna array.

A 28^(th) embodiment provides a non-transitory computer program productcomprising a computer readable medium storing instructions which, whenexecuted on a computer, perform the method of one of the 25^(th) to27^(th) embodiments.

Although some aspects of the described concept have been described inthe context of an apparatus, it is clear that these aspects alsorepresent a description of the corresponding method, where a block or adevice corresponds to a method step or a feature of a method step.Analogously, aspects described in the context of a method step alsorepresent a description of a corresponding block or item or feature of acorresponding apparatus.

Various elements and features of the present invention may beimplemented in hardware using analog and/or digital circuits, insoftware, through the execution of instructions by one or more generalpurpose or special-purpose processors, or as a combination of hardwareand software. For example, embodiments of the present invention may beimplemented in the environment of a computer system or anotherprocessing system. FIG. 6 illustrates an example of a computer system600. The units or modules as well as the steps of the methods performedby these units may execute on one or more computer systems 600. Thecomputer system 600 includes one or more processors 602, like a specialpurpose or a general purpose digital signal processor. The processor 602is connected to a communication infrastructure 604, like a bus or anetwork. The computer system 600 includes a main memory 606, e.g., arandom access memory (RAM), and a secondary memory 608, e.g., a harddisk drive and/or a removable storage drive. The secondary memory 608may allow computer programs or other instructions to be loaded into thecomputer system 600. The computer system 600 may further include acommunications interface 610 to allow software and data to betransferred between computer system 600 and external devices. Thecommunication may be in the form electronic, electromagnetic, optical,or other signals capable of being handled by a communications interface.The communication may use a wire or a cable, fiber optics, a phone line,a cellular phone link, an RF link and other communications channels 612.

The terms “computer program medium” and “computer readable medium” areused to generally refer to tangible storage media such as removablestorage units or a hard disk installed in a hard disk drive. Thesecomputer program products are means for providing software to thecomputer system 600. The computer programs, also referred to as computercontrol logic, are stored in main memory 606 and/or secondary memory608. Computer programs may also be received via the communicationsinterface 610. The computer program, when executed, enables the computersystem 600 to implement the present invention. In particular, thecomputer program, when executed, enable processor 602 to implement theprocesses of the present invention, such as any of the methods describedherein. Accordingly, such a computer program may represent a controllerof the computer system 600. Where the disclosure is implemented usingsoftware, the software may be stored in a computer program product andloaded into computer system 600 using a removable storage drive, aninterface, like communications interface 610.

The implementation in hardware or in software may be performed using adigital storage medium, for example cloud storage, a floppy disk, a DVD,a Blue-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory,having electronically readable control signals stored thereon, whichcooperate (or are capable of cooperating) with a programmable computersystem such that the respective method is performed. Therefore, thedigital storage medium may be computer readable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention may be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier. Inother words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier (or a digital storage medium, or a computer-readable medium)comprising, recorded thereon, the computer program for performing one ofthe methods described herein. A further embodiment of the inventivemethod is, therefore, a data stream or a sequence of signalsrepresenting the computer program for performing one of the methodsdescribed herein. The data stream or the sequence of signals may forexample be configured to be transferred via a data communicationconnection, for example via the Internet. A further embodiment comprisesa processing means, for example a computer, or a programmable logicdevice, configured to or adapted to perform one of the methods describedherein. A further embodiment comprises a computer having installedthereon the computer program for performing one of the methods describedherein.

In some embodiments, a programmable logic device (for example a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods may be performed by any hardware apparatus.

Yet further embodiments are now described.

-   A 1^(st) embodiment provides a transmitter, comprising:    -   an antenna array (202, 304) having a plurality of antennas for a        wireless communication with one or more receivers; and    -   a precoder (204) connected to the antenna array (202, 304), the        precoder (204) to apply a set of beamforming weights to the        antenna array (202, 304), the set of beamforming weights        selected from a codebook (212) to form by the antenna array        (202, 304) one or more transmit/receive beams or nulls pointing        in selected directions;    -   wherein the codebook (212) includes a plurality of sets of        beamforming weights for a plurality of directions,    -   wherein the beamforming weights in the codebook (212) are based        on a first antenna array response matrix (A(φ, θ), A_((h))(φ,        θ), A_((v))(φ, θ)), the first antenna array response matrix        (A(φ, θ), A_((h))(φ, θ), A_((v))(φ, θ)) defined by a second        antenna array response matrix (D(φ, θ)) and one or more        characteristic matrices (G, G_((h)), G_((v))), and    -   wherein the first antenna array response matrix (A(φ, θ),        A_((h))(φ, θ), A_((v))(φ, θ)) contains, for a plurality of        directions, first array response vectors of the antenna array        (202, 304), the second antenna array response matrix (D(φ, θ))        contains, for one or more of the plurality of directions, second        array response vectors of another antenna array, the other        antenna array being different from the antenna array (202, 304),        and the one or more characteristic matrices (G, G_((h)),        G_((v))) describing one or more characteristics of the antenna        array (202, 304).-   A 2^(nd) embodiment provides the transmitter of the 1^(st)    embodiment, wherein    -   the one or more characteristic matrices (G, G_((h)), G_((v)))        are based on a model of the antenna array (202, 304) or on a        measured antenna array response matrix containing, for some or        all of the plurality of directions, measured array response        vectors determined from a measurement of the antenna array (202,        304).-   A 3^(rd) embodiment provides the transmitter of the 2^(nd)    embodiment, wherein    -   the model of the antenna array (202, 304) or the measurement of        the antenna array (202, 304) considers a directional and        non-identical nature of one or more of the respective antennas        in the antenna array (202, 304), and/or an electromagnetic        coupling between the respective antennas in the antenna array        (202, 304), and/or other practical imperfections in the antenna        array (202, 304).-   A 4^(th) embodiment provides the transmitter of the 2^(nd)    embodiment or of the 3^(rd) embodiment, wherein    -   the model of the antenna array (202, 304) or the measurement of        the antenna array (202, 304) describes one or more radiation        characteristics of one or more of the antennas of the antenna        array (202, 304), the radiation characteristics including one or        more of a phase and an amplitude due to a position of an antenna        in the antenna array (202, 304), and/or a coupling imbalance,        and/or a manufacturing imbalance.-   A 5^(th) embodiment provides the transmitter of any one of the    1^(st) to 4^(th) embodiments, wherein    -   the other antenna array comprises a 1-dimensional antenna array,        or a 2-dimensional antenna array or a 3-dimensional antenna        array.-   A 6^(th) embodiment provides the transmitter of any one of the    1^(st) to 5^(th) embodiments, wherein    -   the second antenna array response matrix (D(φ, θ)) is defined by        a 2D-DFT based matrix.-   A 7^(th) embodiment provides the transmitter of any one of the    1^(st) to 6^(th) embodiments, wherein    -   the first antenna array response matrix (A(φ, θ), A_((h))(φ, θ),        A_((v))(φ, θ)) is modeled using the second antenna array        response matrix (D(φ, θ)) and the one or more characteristic        matrices (G, G_((h)), G_((v))).-   An 8^(th) embodiment provides the transmitter of any one of the    1^(st) to 7^(th) embodiments, wherein    -   the codebook (212) is obtained from a first codebook (208)        modified using the one or more characteristic matrices (G,        G_((h)), G_((v))), the first codebook (208) defined by the        second array response vectors of the second antenna array        response matrix (D(φ, θ)).-   A 9^(th) embodiment provides the transmitter of any one of the    1^(st) to 8^(th) embodiments, wherein    -   the first antenna array response matrix comprises a first        antenna array response submatrix (A_((h))(φ, θ)) and a second        antenna array response submatrix (A_((v))(φ, θ)), the first        antenna array response submatrix (A_((h))(φ, θ)) contains, for        the plurality of directions, the array response vectors of all        antenna ports in the antenna array (202, 304) with respect to        the horizontal polarization of an wavefront, and    -   the second antenna array response submatrix (A_((v)), θ))        contains, for the same directions as in the first submatrix        (A_((h))(φ, θ)), the array response vectors of all antenna ports        in the antenna array (202, 304) with respect to the vertical        polarization of the wavefront.-   A 10^(th) embodiment provides the transmitter of the 9^(th)    embodiment, comprising:    -   a first characteristic matrix (G_((h))) corresponding to the        horizontal polarization, the first antenna array response        submatrix (A_((h))(φ, θ)) modeled by the second array response        vectors of the second antenna array response matrix (D(φ, θ))        and the first characteristic matrix (G_((h))), and    -   a second characteristic matrix (G_((v))) corresponding to the        vertical polarization, the second antenna array response        submatrix (A_((v))(φ, θ)) modeled by the second array response        vectors of the second antenna array response matrix (D(φ, θ))        and the second characteristic matrix (G_((v))).-   An 11^(th) embodiment provides the transmitter of the 10^(th)    embodiment, wherein:    -   the codebook (212) comprises a polarimetric codebook, the        polarimetric codebook containing a first sub-codebook (Ω_((h)))        and a second sub-codebook (Ω_((v))),    -   the first sub-codebook (Ω_((h))) contains beamforming weights        for all antenna ports with respect to the horizontal        polarization, the beamforming weights based on the first antenna        array response submatrix (A_((h))(φ, θ)), and the second        sub-codebook (Ω_((v)))) contains beamforming weights for all        antenna ports with respect to the vertical polarization, the        beamforming weights based on the second antenna array response        submatrix (A_((v))(φ, θ)).-   A 12^(th) embodiment provides the transmitter of any one of the    1^(st) to 11^(th) embodiments, wherein    -   the transmitter is configured to transmit the one or more        characteristic matrices (G, G_((h)), G_((v))) to the one or more        receivers as part of control information (504) for the receiver.-   A 13^(th) embodiment provides the transmitter of the 12^(th)    embodiment, wherein    -   the transmitter is configured to transmit one or more design        parameters of the codebook (208) of the other antenna array to        the receiver side.-   A 14^(th) embodiment provides the transmitter of any one of the    1^(st) to 13^(th) embodiments, wherein    -   the antenna array (202, 304) comprises a 1-dimensional antenna        array, or a 2-dimensional antenna array or a 3-dimensional        antenna array, and wherein    -   the 2-dimensional antenna array or the 3-dimensional antenna        array controls the radio wave for beamforming both in the        vertical direction and the horizontal direction, the precoder        (204) having a dual-stage structure, the first stage containing        the sets of beamforming weights based on the codebook (212) to        be applied to each antenna of the antenna array (202, 304), the        second stage containing coefficients that select and/or linearly        combine the beams defined by the first stage to obtain a desired        beam pattern.

A 15^(th) embodiment provides a receiver, comprising:

-   -   an antenna (202, 304) for a wireless communication with a        transmitter; and    -   a signal processor (302, 306, 306) to receive and process a        radio signal received at the antenna via a radio channel (400);    -   wherein the receiver is configured to construct a codebook (212)        including a plurality of sets of beamforming weights for a        plurality of directions, the beamforming weights in the codebook        (212) being based on a first antenna array response matrix    -   (A(φ, θ), A_((h))(φ, θ), A_((v))(φ, θ)) of an antenna array        (202, 304) of the transmitter, and the transmitter selecting a        set of beamforming weights from the codebook (212) to form by        the antenna array (202, 304) a transmit/receive beam pointing in        a selected direction,    -   wherein the receiver is configured to construct the codebook        (212) using a second antenna array response matrix (D(φ, θ)) and        one or more characteristic matrices    -   (G, G_((h))), G_((v)) received from the transmitter, and    -   wherein the first antenna array response matrix (A(φ, θ),        A_((h))(φ, θ), A_((v))(φ, θ)) contains, for a plurality of        directions, first array response vectors of the antenna array        (202, 304), the second antenna array response matrix (D(φ, θ))        contains, for one or more of the plurality of directions, second        array response vectors of another antenna array, the other        antenna array being different from the transmitter's antenna        array (202, 304), and the one or more characteristic matrices        (G, G_((h))), G_((v))) describing one or more characteristics of        the transmitter's antenna array (202, 304).

-   A 16^(th) embodiment provides the receiver of the 15^(th)    embodiment, wherein the receiver is configured for a wireless    communication with a transmitter of any one of the 1^(st) to 14^(th)    embodiments.

-   A 17^(th) embodiment provides the receiver of the 15^(th) embodiment    or of the 16^(th) embodiment, comprising:    -   a channel estimator to estimate a state of the radio channel,    -   wherein the receiver is configured to determine, based on the        radio channel estimation and the constructed codebook (212), a        feedback (500) to the transmitter, the transmitter selecting, on        the basis of the feedback, a set of beamforming weights from the        codebook (212) to form the transmit/receive beam, and    -   wherein the feedback includes control signals on the basis of        which the transmitter selects a set of beamforming weights, or        the feedback transmits a set of beamforming weights from the        receiver to the transmitter.

-   An 18^(th) embodiment provides a wireless communication network,    comprising:    -   a transmitter of any one of the 1^(st) to 14^(th) embodiments;        and    -   one or more receivers of any one of the 15^(th) to 17^(th)        embodiments.

-   A 19^(th) embodiment provides the wireless communication network of    the 18^(th) embodiment, wherein    -   the transmitter comprises a base station serving a user        equipment, or a user equipment served by a base station, or    -   the receiver comprises a base station serving a user equipment,        or a user equipment served by a base station.

-   A 20^(th) embodiment provides a method, comprising:    -   determining a codebook (212) used by a precoder (204) of a        transmitter, the transmitter including an antenna array (202,        304) having a plurality of antennas for a wireless communication        with one or more receivers, and the precoder (204) connected to        the antenna array (202, 304), the codebook (212) including a        plurality of sets of beamforming weights for a plurality of        directions, the beamforming weights in the codebook (212) based        on a first antenna array response matrix (A(φ, θ), A_((h))(φ,        θ)^(, A) _((v))(φ, θ)),    -   wherein determining a codebook (212) includes obtaining the        first antenna array response matrix (A(φ, θ), A_((h))(φ, θ),        A_((v))(φ, θ)) using a second antenna array response matrix        (D(φ, θ)) and one or more characteristic matrices (G, G_((h)),        G_((v))), and    -   wherein the first antenna array response matrix (A(φ, θ),        A_((h))(φ, θ), A_((v))(φ, θ)) contains, for a plurality of        directions, first array response vectors of the antenna array        (202, 304), the second antenna array response matrix (D(φ, θ))        contains, for one or more of the plurality of directions, second        array response vectors of another antenna array, the other        antenna array being different from the antenna array (202, 304),        and the one or more characteristic matrices (G, G_((h)),        G_((v))) describing one or more characteristics of the antenna        array (202, 304).

-   A 21^(st) embodiment provides a method, comprising:    -   receiving and processing, at a receiver having an antenna for a        wireless communication with a transmitter, a radio signal        received at the antenna via a radio channel, the radio signal        including one or more characteristic matrices (G, G_((h)),        G_((v))); and    -   constructing a codebook (212) at the receiver, the codebook        (212) including a plurality of sets of beamforming weights for a        plurality of directions, the beamforming weights in the codebook        (212) being based on a first antenna array response matrix (A(φ,        θ), A_((h))(φ, θ), A_((v))(φ, θ)) of an antenna array (202, 304)        of the transmitter, and the transmitter selecting a set of        beamforming weights from the codebook (212) to form by an        antenna array (202, 304) a transmit/receive beam pointing in a        selected direction,    -   wherein the codebook (212) is constructed using a second antenna        array response matrix (D(φ, θ)) and the one or more        characteristic matrices (G, G_((h)), G_((v))) received from the        transmitter, and    -   wherein the first antenna array response matrix (A(φ, θ),        A_((h))(φ, θ), A_((v))(φ, θ)) contains, for a plurality of        directions, first array response vectors of the antenna array        (202, 304), the second antenna array response matrix (D(φ, θ))        contains, for one or more of the plurality of directions, second        array response vectors of another antenna array, the other        antenna array being different from the transmitter's antenna        array (202, 304), and the one or more characteristic matrices        (G, G_((h)), G_((v))) describing one or more characteristics of        the transmitter's antenna array (202, 304).

-   A 22^(nd) embodiment provides non-transitory computer program    product comprising a computer readable medium storing instructions    which, when executed on a computer, perform the method of the    20^(th) embodiment or of the 21^(st) embodiment.

The above described embodiments are merely illustrative for theprinciples of the present invention. It is understood that modificationsand variations of the arrangements and the details described herein willbe apparent to others skilled in the art. It is the intent, therefore,to be limited only by the scope of the impending patent claims and notby the specific details presented by way of description and explanationof the embodiments herein.

REFERENCES

-   [1] Erik Dahlman, Stefan Parkvall, Johan SkOld, “4G:    LTE/LTE-Advanced for Mobile Broadband,” Academic Press, 2011.    (ISBN:012385489X 9780123854896)-   [2] 3GPP TS 36.211 V10.7.0, “3rd Generation Partnership Project;    Technical Specification Group Radio Access Network; Evolved    Universal Terrestrial Radio Access (E-UTRA); Physical Channels and    Modulation (Release 10),” February 2013.-   [3] 3GPP TR 36.897 V13.0.0, “3rd Generation Partnership Project;    Technical Specification Group Radio Access Network; Study on    elevation beamforming/Full-Dimension (FD) Multiple Input Multiple    Output (M IMO) for LTE (Release 13),” Jun. 2015.-   [4] Cheng et al., “Two-dimensional Discrete Fourier Transform based    Codebook for Elevation Beamforming,” United States Patent    Application, US2016/0173180 A1, Jun. 2016.-   [5] Buhren M Pesavento M., and Bohme J. F., “Virtual array design    for array interpolation using differential geometry,” In    international conference on Acoustics, Speech. and Signal    Processing, 2004.-   [6] Mathews, Cherian P., and Michael D. Zoltowski. “Eigen structure    techniques for 2-D angle estimation with uniform circular arrays,”    IEEE Transactions on signal processing, Vol. 42, no. 9, PP.    2395-2407, 1994.-   [7] Florian Roemer, Mohamed Ibrahim, Roman Alieiev, Markus Landmann,    Reiner S. Thoma, and Giovanni Del Galdo, “Polarimetric Compressive    Sensing Based DOA Estimation,” Workshop on Smart Antennas, Mar.    2014.-   [8] M. Landmann, and R. Thoma, “Common Pitfalls in Multidimensional    High Resolution Channel Parameter Estimation”, IEEE Digital Signal    Processing Workshop, Sep. 2009.

The invention claimed is:
 1. A transmitter, comprising: an antenna arraycomprising a plurality of antennas for a wireless communication with oneor more receivers; and a precoder connected to the antenna array, theprecoder to apply a set of beamforming weights to the antenna array, theset of beamforming weights selected from a codebook to form by theantenna array one or more transmit/receive beams or nulls pointing inselected directions; wherein the codebook comprises a plurality of setsof beamforming weights for a plurality of directions, wherein thebeamforming weights in the codebook are based on a first antenna arrayresponse matrix, the first antenna array response matrix defined by asecond antenna array response matrix and one or more characteristicmatrices, wherein the first antenna array response matrix comprises, fora plurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix comprises, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from theantenna array, and the one or more characteristic matrices describingone or more characteristics of the antenna array, and wherein the firstantenna array response matrix is modeled using the second antenna arrayresponse matrix and the one or more characteristic matrices, each of thefirst array response vectors being a product of a characteristic matrixand a corresponding second array response vector.
 2. The transmitter ofclaim 1, wherein the one or more characteristic matrices are based on amodel of the antenna array or on a measured antenna array responsematrix comprising, for some or all of the plurality of directions,measured array response vectors determined from a measurement of theantenna array.
 3. The transmitter of claim 2, wherein the model of theantenna array or the measurement of the antenna array considers adirectional and non-identical nature of one or more of the respectiveantennas in the antenna array, or an electromagnetic coupling betweenthe respective antennas in the antenna array, or other practicalimperfections in the antenna array.
 4. The transmitter of claim 2,wherein the model of the antenna array or the measurement of the antennaarray describes one or more radiation characteristics of one or more ofthe antennas of the antenna array, the radiation characteristicscomprising one or more of a phase and an amplitude due to a position ofan antenna in the antenna array, or a coupling imbalance, or amanufacturing imbalance.
 5. The transmitter of claim 1, wherein theother antenna array comprises a 1-dimensional antenna array, or a2-dimensional antenna array or a 3-dimensional antenna array.
 6. Thetransmitter of claim 1, wherein the second antenna array response matrixis defined by a 2-dimensional discrete Fourier transform, 2D-DFT, basedmatrix.
 7. The transmitter of claim 1, wherein the codebook is acquiredfrom a first codebook modified using the one or more characteristicmatrices, the first codebook defined by the second array responsevectors of the second antenna array response matrix.
 8. The transmitterof claim 1, wherein the first antenna array response matrix comprises afirst antenna array response submatrix and a second antenna arrayresponse submatrix, the first antenna array response submatrixcomprises, for the plurality of directions, the array response vectorsof all antenna ports in the antenna array with respect to the horizontalpolarization of a wavefront, and the second antenna array responsesubmatrix comprises, for the same directions as in the first submatrix,the array response vectors of all antenna ports in the antenna arraywith respect to the vertical polarization of the wavefront.
 9. Thetransmitter of claim 8, comprising: a first characteristic matrixcorresponding to the horizontal polarization, the first antenna arrayresponse submatrix modeled by the second array response vectors of thesecond antenna array response matrix and the first characteristicmatrix, and a second characteristic matrix corresponding to the verticalpolarization, the second antenna array response submatrix modeled by thesecond array response vectors of the second antenna array responsematrix and the second characteristic matrix.
 10. The transmitter ofclaim 9, wherein: the codebook comprises a polarimetric codebook, thepolarimetric codebook comprising a first sub-codebook and a secondsub-codebook, the first sub-codebook comprises beamforming weights forall antenna ports with respect to the horizontal polarization, thebeamforming weights based on the first antenna array response submatrix,and the second sub-codebook comprises beamforming weights for allantenna ports with respect to the vertical polarization, the beamformingweights based on the second antenna array response submatrix.
 11. Thetransmitter of claim 1, wherein the transmitter is configured totransmit the one or more characteristic matrices to the one or morereceivers as part of control information for the receiver.
 12. Thetransmitter of claim 11, wherein the transmitter is configured totransmit one or more design parameters of the codebook of the otherantenna array to the receiver side.
 13. The transmitter of claim 1,wherein the antenna array comprises a 1-dimensional antenna array, or a2-dimensional antenna array or a 3-dimensional antenna array, andwherein the 2-dimensional antenna array or the 3-dimensional antennaarray controls the radio wave for beamforming both in the verticaldirection and the horizontal direction, the precoder comprising adual-stage structure, the first stage comprising the sets of beamformingweights based on the codebook to be applied to each antenna of theantenna array, the second stage comprising coefficients that select orlinearly combine the beams defined by the first stage to acquire adesired beam pattern.
 14. A receiver, comprising: an antenna for awireless communication with a transmitter; and a signal processor toreceive and process a radio signal received at the antenna via a radiochannel; wherein the receiver is configured to construct a codebookcomprising a plurality of sets of beamforming weights for a plurality ofdirections, the beamforming weights in the codebook being based on afirst antenna array response matrix of an antenna array of thetransmitter, and the transmitter selecting a set of beamforming weightsfrom the codebook to form by the antenna array a transmit/receive beampointing in a selected direction, wherein the receiver is configured toreceive from the transmitter one or more characteristic matrices and toconstruct the codebook using a second antenna array response matrix andthe one or more characteristic matrices received from the transmitter,and wherein the first antenna array response matrix comprises, for aplurality of directions, first array response vectors of the antennaarray, the second antenna array response matrix comprises, for one ormore of the plurality of directions, second array response vectors ofanother antenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors.
 15. The receiver of claim 14, wherein the receiver isconfigured for a wireless communication with a transmitter of claim 1.16. The receiver of claim 14, comprising: a channel estimator toestimate a state of the radio channel, wherein the receiver isconfigured to determine, based on the radio channel estimation and theconstructed codebook, a feedback to the transmitter, the transmitterselecting, on the basis of the feedback, a set of beamforming weightsfrom the codebook to form the transmit/receive beam, and wherein thefeedback comprises control signals on the basis of which the transmitterselects a set of beamforming weights, or the feedback transmits a set ofbeamforming weights from the receiver to the transmitter.
 17. A wirelesscommunication network, comprising: a transmitter, comprising: an antennaarray comprising a plurality of antennas for a wireless communicationwith one or more receivers; and a precoder connected to the antennaarray, the precoder to apply a set of beamforming weights to the antennaarray, the set of beamforming weights selected from a codebook to formby the antenna array one or more transmit/receive beams or nullspointing in selected directions; wherein the codebook comprises aplurality of sets of beamforming weights for a plurality of directions,wherein the beamforming weights in the codebook are based on a firstantenna array response matrix, the first antenna array response matrixdefined by a second antenna array response matrix and one or morecharacteristic matrices, wherein the first antenna array response matrixcomprises, for a plurality of directions, first array response vectorsof the antenna array, the second antenna array response matrixcomprises, for one or more of the plurality of directions, second arrayresponse vectors of another antenna array, the other antenna array beingdifferent from the antenna array, and the one or more characteristicmatrices describing one or more characteristics of the antenna array,and wherein the first antenna array response matrix is modeled using thesecond antenna array response matrix and the one or more characteristicmatrices, each of the first array response vectors being a product of acharacteristic matrix and a corresponding second array response vectors;and at least one receiver comprising: an antenna for a wirelesscommunication with a transmitter; and a signal processor to receive andprocess a radio signal received at the antenna via a radio channel;wherein the receiver is configured to construct a codebook comprising aplurality of sets of beamforming weights for a plurality of directions,the beamforming weights in the codebook being based on a first antennaarray response matrix of an antenna array of the transmitter, and thetransmitter selecting a set of beamforming weights from the codebook toform by the antenna array a transmit/receive beam pointing in a selecteddirection, wherein the receiver is configured to receive from thetransmitter one or more characteristic matrices and to construct thecodebook usinq a second antenna array response matrix and the one ormore characteristic matrices received from the transmitter, and whereinthe first antenna array response matrix comprises, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix comprises, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vector.
 18. The wireless communication network of claim 17,wherein the transmitter comprises a base station serving a userequipment, or a user equipment served by a base station, or the receivercomprises a base station serving a user equipment, or a user equipmentserved by a base station.
 19. A method, comprising: determining acodebook used by a precoder of a transmitter, the transmitter comprisingan antenna array comprising a plurality of antennas for a wirelesscommunication with one or more receivers, and the precoder connected tothe antenna array, the codebook comprising a plurality of sets ofbeamforming weights for a plurality of directions, the beamformingweights in the codebook based on a first antenna array response matrix,wherein determining a codebook comprises acquiring the first antennaarray response matrix using a second antenna array response matrix andone or more characteristic matrices, and wherein the first antenna arrayresponse matrix comprises, for a plurality of directions, first arrayresponse vectors of the antenna array, the second antenna array responsematrix comprises, for one or more of the plurality of directions, secondarray response vectors of another antenna array, the other antenna arraybeing different from the antenna array, and the one or morecharacteristic matrices describing one or more characteristics of theantenna array, and wherein the first antenna array response matrix ismodeled using the second antenna array response matrix and the one ormore characteristic matrices, each of the first array response vectorsbeing a product of a characteristic matrix and a corresponding secondarray response vectors.
 20. A method, comprising: receiving andprocessing, at a receiver comprising an antenna for a wirelesscommunication with a transmitter, a radio signal received at the antennavia a radio channel, the radio signal comprising one or morecharacteristic matrices; and constructing a codebook at the receiver,the codebook comprising a plurality of sets of beamforming weights for aplurality of directions, the beamforming weights in the codebook beingbased on a first antenna array response matrix of an antenna array ofthe transmitter, and the transmitter selecting a set of beamformingweights from the codebook to form by an antenna array a transmit/receivebeam pointing in a selected direction, wherein the codebook isconstructed using a second antenna array response matrix and the one ormore characteristic matrices received from the transmitter, wherein thefirst antenna array response matrix comprises, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix comprises, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors.
 21. A non-transitory digital storage medium havingstored thereon a computer program for performing a method, comprising:determining a codebook used by a precoder of a transmitter, thetransmitter comprising an antenna array comprising a plurality ofantennas for a wireless communication with one or more receivers, andthe precoder connected to the antenna array, the codebook comprising aplurality of sets of beamforming weights for a plurality of directions,the beamforming weights in the codebook based on a first antenna arrayresponse matrix, wherein determining a codebook comprises acquiring thefirst antenna array response matrix using a second antenna arrayresponse matrix and one or more characteristic matrices, and wherein thefirst antenna array response matrix comprises, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix comprises, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from the antennaarray, and the one or more characteristic matrices describing one ormore characteristics of the antenna array, and wherein the first antennaarray response matrix is modeled using the second antenna array responsematrix and the one or more characteristic matrices, each of the firstarray response vectors being a product of a characteristic matrix and acorresponding second array response vectors, when said computer programis run by a computer.
 22. A non-transitory digital storage medium havingstored thereon a computer program for performing a method, comprising:receiving and processing, at a receiver comprising an antenna for awireless communication with a transmitter, a radio signal received atthe antenna via a radio channel, the radio signal comprising one or morecharacteristic matrices; and constructing a codebook at the receiver,the codebook comprising a plurality of sets of beamforming weights for aplurality of directions, the beamforming weights in the codebook beingbased on a first antenna array response matrix of an antenna array ofthe transmitter, and the transmitter selecting a set of beamformingweights from the codebook to form by an antenna array a transmit/receivebeam pointing in a selected direction, wherein the codebook isconstructed using a second antenna array response matrix and the one ormore characteristic matrices received from the transmitter, wherein thefirst antenna array response matrix comprises, for a plurality ofdirections, first array response vectors of the antenna array, thesecond antenna array response matrix comprises, for one or more of theplurality of directions, second array response vectors of anotherantenna array, the other antenna array being different from thetransmitter's antenna array, and the one or more characteristic matricesdescribing one or more characteristics of the transmitter's antennaarray, and wherein the first antenna array response matrix is modeledusing the second antenna array response matrix and the one or morecharacteristic matrices, each of the first array response vectors beinga product of a characteristic matrix and a corresponding second arrayresponse vectors, when said computer program is run by a computer.